Memwipe is a non-canon term denoting the elimination of all hate recorded by a mob.
Generally unique to named mobs, and triggered in a number of different ways (e.g., fixed casting time, at a percentage of health, or randomly), a memwipe poses a unique challenge to groups and raids, as the tank no longer has "control" of the mob. Hate being reduced to zero will cause the mob, as if being attacked for the first time, to turn its aggro towards the first subsequent strike.
When an opponent is known to utilize memwipe, the tank (or secondary tank / offtank, if appropriate) should be prepared to deploy hate-increasing or force-to-target spells at the appropriate moment. Further, it may be necessary to coordinate a "stop all attacks" alert for damage-dealing members of the group. This can be particularly important for relatively poorly protected cloth-armored casting classes, who may deliver high damage and not survive the response from the now-hate-reset mob.