Opposite of aggro, or to reduce / remove the amount of threat.
For example:
- If a character runs near a hostile mob in the world, that mob will aggro. If the player runs far enough away from the mob, the mob should deaggro, or reset. (In instances, however, it is difficult or impossible to outrun aggro without leaving the instance.)
- A group of adventurers is preparing to pull a group of mobs. If one of the adventurers runs up and aggros the mobs prematurely, the group may choose to let the puller die rather than engage in combat, since the mobs will deaggro after killing the single player if the rest of the group does not intervene. While the single player must be resurrected, this may be preferable to a full wipe that would occur if the group engaged when not fully buffed / mana'd up.
The term deaggro is most commonly used in the context of resetting the target's threat list altogether. Note that some class abilities (such as the Druid's cower (cat form), Priest's 
[Fade], and Rogue's 
[Feint]) reduce the amount of threat, but do not remove the user from the threat list completely.