Ability Score

Each creature has six ability scores: Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma. These scores represent a creature's most basic attributes. The higher the score, the more raw potential and talent your character possesses.

Action

An action is a discrete measurement of time during a round of combat. Using abilities, casting spells, and making attacks all require actions to perform. There are a number of different kinds of actions, such as a standard action, move action, swift action, free action, and full-round action.

Adventure

An adventure is a self-contained storyline the PCs experience. An adventure is composed of a series of encounters furthering the storyline.

Alignment

Alignment represents a creature's basic moral and ethical attitude. Alignment has two components: one describing whether a creature is lawful, neutral, or chaotic, followed by another that describes whether a character is good, neutral, or evil. Alignments are usually abbreviated using the first letter of each alignment component, such as LN for lawful neutral or CE for chaotic evil. Creatures that are neutral in both components are denoted by a single “N.”

Armor Class (AC)

All creatures in the game have an Armor Class. This score represents how hard it is to hit a creature in combat. As with other scores, higher is better. This is the target number enemies need to hit you. Your basic AC is 10 + Dex modifier + armor bonus + shield bonus + spells or magic items that grant an AC bonus.

Base Attack Bonus (BAB)

Each creature has a base attack bonus and it represents its skill in combat. As a character gains levels or Hit Dice, his base attack bonus improves. When a creature's base attack bonus reaches +6, +11, or +16, he receives an additional attack in combat when he takes a full-attack action (which is one type of full-round action—see Combat).

Bonus

Bonuses are numerical values that are added to checks and statistical scores. Most bonuses have a type, and as a general rule, bonuses of the same type are not cumulative (do not “stack”)—only the greater bonus granted applies.

The important aspect of bonus types is that two bonuses of the same type don't generally stack. With the exception of dodge bonuses, most circumstance bonuses, and racial bonuses, only the better bonus of a given type works. Bonuses without a type always stack, unless they are from the same source.

Campaign

A campaign is a collection of stories weaving into an overreaching narrative. It may be a string of published adventures, a chain of homebrewed material, or an Adventure Path designed to be played as a series. A campaign may or may not have a definitive or predefined end point.

Campaign Arc

A sequence of adventures that mesh well with each other, usually part of a larger campaign. Game Masters often run these shorter arcs to create a story that’s more concise than a full campaign but longer than a single adventure.

Caster Level (CL)

Caster level represents a creature's power and ability when casting spells. When a creature casts a spell, it often contains a number of variables, such as range or damage, that are based on the caster's level.

Caster Level Check

A Caster Level Check is made by rolling 1d20 and adding the caster level of the character or creature casting the spell. Many spells (such as dispel magic, nondetection, planar binding, remove curse, and many more) require or include Caster Level Checks to function.

Character Level

The total level of the character, which is the sum of all class levels held by that character.

Check

A check is a d20 roll which may or may not be modified by another value. The most common types are attack rolls, skill checks, and saving throws.

Class

Classes represent chosen professions taken by characters and some other creatures. Classes give a host of bonuses and allow characters to take actions that they otherwise could not, such as casting spells or changing shape. As a creature gains levels in a given class, it gains new, more powerful abilities. Most PCs gain levels in the core classes or prestige classes, since these are the most powerful. Most NPCs gain levels in NPC Classes, which are less powerful.

Class Level

The level of a character in a particular class. For a character with levels in only one class, class level and character level are the same.

Combat Maneuver

This is an action taken in combat that does not directly cause harm to your opponent, such as attempting to Trip him, Disarm him, or Grapple with him.

Combat Maneuver Bonus (CMB)

Combat Maneuver Bonus represents how skilled a creature is at performing a combat maneuver. When attempting to perform a combat maneuver, this value is added to the character's d20 roll.

Combat Maneuver Defense (CMD)

Combat Maneuver Defense represents how hard it is to perform a combat maneuver against this creature. A creature's CMD is used as the difficulty class when performing a maneuver against that creature.

Concentration Check

When a creature is casting a spell, but is disrupted during the casting, he must make a concentration check or fail to cast the spell.

Core Class

One of the standard classes found in Classes.

Creature

A creature is an active participant in the story or world. This includes PCs, NPCs, and monsters.

Critical Hit

When you make an attack roll and get a natural 20 (the d20 shows 20), you hit regardless of your target's Armor Class, and you have scored a “threat,” meaning the hit might be a critical hit (or “crit”). To find out if it's a critical hit, you immediately make an attempt to “confirm” the critical hit—another attack roll with all the same modifiers as the attack roll you just made. If the confirmation roll also results in a hit against the target's AC, your original hit is a critical hit. (The critical roll just needs to hit to give you a crit, it doesn't need to come up 20 again.) If the confirmation roll is a miss, then your hit is just a regular hit.

Damage

Damage is determined by rolling the dice listed with the weapon. Melee weapons deal their listed damage + Strength modifier. Ranged weapons usually do only their listed damage. Some weapons gain additional bonuses from magic or other effects. Spells do their listed damage.

Damage Reduction (DR)

Creatures that are resistant to harm typically have damage reduction. This amount is subtracted from any damage dealt to them from a physical source. Most types of DR can be bypassed by certain types of weapons. This is denoted by a “/” followed by the type, such as “10/cold iron.” Some types of DR apply to all physical attacks. Such DR is denoted by the “—” symbol. See Special Abilities for more information.

Difficulty Class (DC)

Whenever a creature attempts to perform an action whose success is not guaranteed, he must make some sort of check (usually a skill check). The result of that check must meet or exceed the Difficulty Class of the action that the creature is attempting to perform in order for the action to be successful.

Encounter

An encounter is a short scene in which the PCs are actively doing something. Examples of encounters include a combat with a monster, a social interaction significant to the adventure’s plot, an attempt to disarm a trap, or the discovery of a mystery or clue requiring further investigation.

Extraordinary Abilities (Ex)

Extraordinary abilities are unusual abilities that do not rely on magic to function. They are not something that just anyone can do or even learn to do without extensive training. Effects or areas that suppress or negate magic have no effect on extraordinary abilities.

Experience Points (XP)

As a character overcomes challenges, defeats monsters, and completes quests, he gains experience points. These points accumulate over time, and when they reach or surpass a specific value, the character gains a level.

Feat

A feat is an ability a creature has mastered. Feats often allow creatures to circumvent rules or restrictions. Creatures receive a number of feats based off their Hit Dice, but some classes and other abilities grant bonus feats.

Game Master (GM)

A Game Master is the person who adjudicates the rules and controls all of the elements of the story and world that the players explore. A GM's duty is to provide a fair and fun game. The game’s storyteller, referee, and director.

Hit Dice (HD)

Hit Dice represent a creature's general level of power and skill. As a creature gains levels, it gains additional Hit Dice. Monsters, on the other hand, gain racial Hit Dice, which represent the monster's general prowess and ability. Hit Dice are represented by the number the creature possesses followed by a type of die, such as “3d8.” This value is used to determine a creature's total hit points. In this example, the creature has 3 Hit Dice. When rolling for this creature's hit points, you would roll a d8 three times and add the results together, along with other modifiers.

Hit Points (hp)

Hit points are an abstraction signifying how robust and healthy a creature is at the current moment. To determine a creature's hit points, roll the dice indicated by its Hit Dice. A creature gains maximum hit points if its first Hit Die roll is for a character class level. Creatures whose first Hit Die comes from an NPC class or from his race roll their first Hit Die normally. Wounds subtract hit points, while healing (both natural and magical) restores hit points. Some abilities and spells grant temporary hit points that disappear after a specific duration. When a creature's hit points drop below 0, it becomes unconscious. When a creature's hit points reach a negative total equal to its Constitution score, it dies.

Initiative

Whenever combat begins, all creatures involved in the battle must make an initiative check to determine the order in which creatures act during combat. The higher the result of the check, the earlier a creature gets to act. Initiative is a d20 roll + Dexterity modifier + any bonus modifiers. The higher the result, the earlier you can act.

Level

A character's level represents his overall ability and power. There are three types of levels. Class level is the number of levels of a specific class possessed by a character. Character level is the sum of all of the levels possessed by a character in all of his classes. In addition, spells have a level associated with them numbered from 0 to 9. This level indicates the general power of the spell. As a spellcaster gains levels, he learns to cast spells of a higher level.

Line of Effect

A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight. A line of effect starts from any corner of your square and extends to the limit of its range or until it strikes a barrier that would block it. A line-shaped spell affects all creatures in squares through which the line passes.

Line of Sight

A line of sight is the same as a Line of Effect but with the additional restriction that that it is blocked by fog, darkness, and other factors that limit normal sight (such as Concealment).

Melee Attack

An attack in hand-to-hand combat. A basic melee attack is a d20 roll + base attack bonus + Strength modifier + any related or magical bonuses.

Metagaming

This is when characters act on information that they don’t have access to, but which their players know from the real world. Metagaming comes into play when players fail to maintain a divide between in-character knowledge and out-of-character knowledge. That could include anything from uncannily accurate in-character predictions from a player who’s already read the adventure, players recognizing monsters when their characters wouldn’t, low-Intelligence characters accessing well-educated players’ knowledge and talents, etc.

Monster

Monsters are creatures that rely on racial Hit Dice instead of class levels for their powers and abilities (although some possess class levels as well). PCs are usually not monsters.

Movement

How many feet you can move with a single move action on your turn. Each square on a battle grid represents 5 feet.

Multiplying

When you are asked to apply more than one multiplier to a roll, the multipliers are not multiplied by one another. Instead, you combine them into a single multiplier, with each extra multiple adding 1 less than its value to the first multiple. For example, if you are asked to apply a ×2 multiplier twice, the result would be ×3, not ×4.

Natural 1

A natural 1 (the d20 comes up 1) on an attack roll is always a miss.

Natural 20

A natural 20 (the d20 comes up 20) is always a hit. A natural 20 is also a Critical Threat—a possible critical hit.

Nonplayer Character (NPC)

A character run by the Game Master, such as townsfolk, villains, monsters, and so on.

Penalty

Penalties are numerical values that are subtracted from a check or statistical score. Penalties do not have a type and most penalties stack with one another.

Player Character (Character, PC)

Characters controlled by the players.

Ranged attack

An attack with a projectile weapon, such as a bow and arrow. A basic ranged attack is a d20 roll + base attack bonus + Dexterity modifier + any related or magical bonuses .

Rest

Round

Combat is measured in rounds. During an individual round, all creatures have a chance to take a turn to act, in order of initiative. A round represents 6 seconds in the game world.

Rounding

Occasionally the rules ask you to round a result or value. Unless otherwise stated, always round down. For example, if you are asked to take half of 7, the result would be 3.

Saving Throw

When a creature is the subject of a dangerous spell or effect, it often receives a saving throw to mitigate the damage or result. Saving throws are passive, meaning that a character does not need to take an action to make a saving throw—they are made automatically. There are three types of saving throws: Fortitude (used to resist poisons, diseases, and other bodily ailments), Reflex (used to avoid effects that target an entire area, such as fireball), and Will (used to resist mental attacks and spells).

Session

A session is a single bout of gaming. Not every session ties up an adventure; many adventures require multiple sessions to complete. The duration of sessions varies from group to group, from a few hours to a weekend.

Skill

A skill represents a creature's ability to perform an ordinary task, such as climb a wall, sneak down a hallway, or spot an intruder. The number of ranks possessed by a creature in a given skill represents its proficiency in that skill. As a creature gains Hit Dice, it also gains additional skill ranks that can be added to its skills.

Skill Check

Your ability to do something, from lying to climbing a wall or healing wounds. A skill check is a d20 roll + your skill modifier from the related skill (if any).

Spell

Spells can perform a wide variety of tasks, from harming enemies to bringing the dead back to life. Spells specify what they can target, what their effects are, and how they can be resisted or negated.

Spell-Like Abilities (Sp)

Spell-like abilities function just like spells, but are granted through a special racial ability or by a specific class ability (as opposed to spells, which are gained by spellcasting classes as a character gains levels).

Spell Resistance (SR)

Some creatures are resistant to magic and gain spell resistance. When a creature with spell resistance is targeted by a spell, the caster of the spell must make a caster level check to see if the spell affects the target. The DC of this check is equal to the target creature's SR (some spells do not allow SR checks).

Stacking

Stacking refers to the act of adding together bonuses or penalties that apply to one particular check or statistic. Generally speaking, most bonuses of the same type do not stack. Instead, only the highest bonus applies. Most penalties do stack, meaning that their values are added together. Penalties and bonuses generally stack with one another, meaning that the penalties might negate or exceed part or all of the bonuses, and vice versa.

Supernatural Abilities (Su)

Supernatural abilities are magical attacks, defenses, and qualities. These abilities can be always active or they can require a specific action to utilize. The supernatural ability's description includes information on how it is used and its effects.

Take 10

When a character or creature is not in immediate danger or distracted, it may choose to take 10 on some rolls (specifically, skill checks). Instead of rolling 1d20 for the check, calculate the result as if the die had rolled a 10. For many routine tasks, taking 10 makes them automatically successful. Distractions or threats (such as combat) make it impossible to take 10. In most cases, taking 10 is purely a safety measure—you know (or expect) that an average roll will succeed but fear that a poor roll might fail, so you elect to settle for the average roll (a 10). Taking 10 is especially useful in situations where a particularly high roll wouldn't help.

Take 20

When a character or creature has plenty of time, and is not faced with threats or distractions, and the skill being attempted carries no penalties for failure, he/it can take 20. Instead of rolling 1d20 for the check, just calculate the result as if the die had rolled a 20.

Taking 20 means you continue trying until you get it right, and assumes that you will fail many times before succeeding. Taking 20 takes 20 times as long as making a single check would take (usually 2 minutes for a skill that takes 1 round or less to perform).

Since taking 20 assumes that your character will fail many times before succeeding, your character would automatically incur any penalties for failure before he or she could complete the task (hence why it is generally not allowed with skills that carry such penalties).

TPK

TPK stands for "Total Party Kill" a sometimes accidental result of a poorly designed encounter by the GM, or poor decision making by the players, a combination of the two, or simply, bad luck and dice rolling. This definition provided by your friendly neighborhood d20pfsrd.com collaborators :)

Turn

In a round, a creature receives one turn, during which it can perform a wide variety of actions. Generally in the course of one turn, a character can perform one standard action, one move action, one swift action, and a number of free actions. Less-common combination's of actions are permissible as well, see Combat for more details.