Tap

Tapped is a state that occurs when a player damages a mob, locking its experience gain, loot and corpse to be available only to that player or anyone in that player's party. Another term for a tap is a "tag".

This state is indicated by the mob's health bar turning gray.

Attacks by a permanent pet that can be auto-cast will not tap a mob. This is to prevent players from farming low-level mobs while AFK by relying on their pets to automatically target mobs. One could argue that this should not apply when the player has commanded the pet to attack the specific mob; however, if this exception existed, it could be abused with a macro such as:

/petattack [@pettarget,harm]

Temporary pets or guardians, such as those created by Mirror Image, do tap mobs.

Abilities that don't deal damage can tap. This means that for very high-level mages and very low-level mobs, farming using only Spellsteal and Molten Armor is possible, and has the advantage that the corpses land within the mage's melee range for more convenient looting.

Trying to use an item on a mob that has been tagged by someone else or someone from outside their party will return the error message "Target is tapped." However, players can still attack and kill the tapped mob even if they were not the ones to tap it.

It is possible for one player to tag multiple mobs in order to keep others from doing so. Although controversial, it is not against Blizzard's Rules of Conduct to tag multiple mobs.

Players can't be tapped. Whenever a player dies while in combat with another player, all nearby living enemy players of low enough level earn an honorable kill, even if they didn't participate in the fight and even if most of the damage was caused by a mob.

Even if a mob is tapped, you won't get credit for the kill if most of the damage was caused by an NPC. Thus, low-level players can't get loot from high-level mobs by pulling them to guards.

For a quest where the objective is to kill a named mob, tapping won't prevent the quest objective from being marked complete for other players who dealt damage to it.