To end a client program's connection to a server, for a number of computer services. While it's possible for the disconnection to be accidental, the more common usage involves a deliberate act on the part of someone with privileges on the server. The classic examples for today's inhabitants of Cyberia involve small, individual-run servers with obnoxious clients, such as IRC and various game servers - an older generation might think of MUCKs or BBSes.

Example usage: "Yeah, so we were chatting on #randomchannel when this idiot came in complaining about how he had been booted from the Return to Castle Wolfenstein server for being a team killer - so the op booted his ass again."