[common on IRC, MUD and among gamers; from “robot”]
1. An IRC or MUD user who is actually a program. On IRC, typically the robot provides some useful service. Examples are NickServ, which tries to prevent random users from adopting nicks already claimed by others, and MsgServ, which allows one to send asynchronous messages to be delivered when the recipient signs on. Also common are ‘annoybots’, such as KissServ, which perform no useful function except to send cute messages to other people. Service bots are less common on MUDs; but some others, such as the ‘Julia’ bot active in 1990--91, have been remarkably impressive Turing-test experiments, able to pass as human for as long as ten or fifteen minutes of conversation.
2. An AI-controlled player in a computer game (especially a first-person shooter such as Quake) which, unlike ordinary monsters, operates like a human-controlled player, with access to a player's weapons and abilities. An example can be found at http://www.telefragged.com/thefatal/.
3. Term used, though less commonly, for a web spider. The file for controlling spider behavior on your site is officially the “Robots Exclusion File” and its URL is “http://<somehost>/robots.txt”)
Note that bots in all senses were ‘robots’ when the terms first appeared in the early 1990s, but the shortened form is now habitual.