A MMORPG term which stands for Mesmerize. Mesmerize works as a hypnotic trance that is cast by magic upon a mob or enemy player. This spell causes the target (multiple targets in the case of an AoE spell) to be unable to move or react to game stimuli. For the length of the mez, they are as still as trees. UNLESS someone breaks the mez by causing damage to the mesmerized target, at which point the target is free from the effects of the spell and can react normally -- which usually means taking out the squishy that mezzed everyone... ;)
When one is in the throes of a mez spell, one can do nothing but think about what they would otherwise be doing and stand there embarassed at being caught in a mez in the first place. Some games have mechanisms whereby a player can cast a counter-spell that mitigates the effects of the mez, such as a Purge of some sort.