A melee range combat maneuver used from hiding. The attack is more often used to for training rather than for offensive purposes.
Thieves have a number of special ambushes of their own. Most, however, don't work on flying targets. Ambush commands are listed with AMBUSH HELP.
Originally the attack needed to be preceded by the AMBUSH command(e.g. AMBUSH SLICE), but now you just need to enter the desired attack and the game will default to Ambush if you're in hiding.