Mezzed (MMO?) / Dazed: Target is unable to act at all until attacked

Stun: Target is unable to react at all till duration of stun is over.

Root: Target is unable to move from position

Grappled: Target is held down, unable to make physical actions, but can still speak and has presence of mind.

Heal over Time (HoT): Healing over a period of time, as opposed to a sudden burst

Damage over Time (DoT): Damage over a period of time, as opposed to a sudden burst. Example: posion

Knock-down: Knocked down to the floor. Usually incur a penalty to actions

Knock-back: Knocked away from current position, does not automatically implies a knock-down

Buff: Usually some sort of benefical state changes to the target

Debuff: Usually some sort of negative state changes to the target

Response: Able to activate an ability in response to an event (a critical hit, suffers a critical hit, on evading an enemy etc.)

Burst/Nuke: Usually refers to a one-time huge amount of damage

Area of Effect (AoE): Attack that affects a radius

To-Hit: Related to skill-check, chance of hitting in combat

Passive Defence: A To-Hit roll against a single value not modified by dice rolls (D&D 3.5E, 4E)

Active Defence: To Hit roll against a dice roll

Save: Usually a condition to a debuff, roll a stat/skill vs. a target number or else the debuff will go off.

Delayed Debuff: Debuff with a timer on it

Tank: Front-line combatant who usually soak up damage and protects back row protectors.

Skill Reuse Clause: Some skill, usually used to observe the surroundings or divinations, has a reuse clause. Example: Warlock’s Appraise skill gives the same results upon reuse on the same target for Dragon Warriors.

Hero Points: A form of meta-game currency which allows player to gain out-of-game exceptions, such as extra turns, additional dice roll, introduce minor facts into the game (Action Points in D&D Eberron, Fate Points in Fate, Fortune points in Warhammer FRPG)

Taunt: Forces an opponent to attack the taunting character

Condition: (First saw in Weapons of the God?) Imposes a condition on a character. If he obeys the condition, he may get a buff; disobedies, and he get a debuff.

Meta-Game Currency: Ways in which GM and players can influence the plot and outcomes using some form of karma. Exceptional success on the GM’s side gives currency to the players; players may spend the currency as a form of Hero Points. Usually, there is a condition to give such currency to the GM, such as on a failure of the player. (See in Don’t Rest your Head)

Crafting Skills/Abilities: Rules which allows players to create their own items. Usually, the items have capacity which outstrips the player’s own (D&D, Dragon Warriors)

Accumulation: Allow players to store certain form of currency to ‘charge-up’ attacks or for abilities. Usually, there is a condition to accumulate the currency and using those abilities will drain the currency. Other variants include needing a number of one type of currency for an abilitiy as a prerequsite. (Iron Heroes, Champions and Rune Keepers in Lord of the Rings Online).

Friendly Fire: Usually a AoE damage/debuff that does damage to allies and enemies irregardless.

Cooldown (MMO?): A time restriction to certain abilities use; D&D 3.5E has x numbers of uses per day, D&D 4E has per-encounter, at-will and etc.