An acronym for "Away From Keyboard", meaning a user is not at their computer. There is a command, /afk, which marks the user as AFK. The name of the character will show up as <AFK>Username. You can also type a message after /afk which will be sent to anyone who sends you a tell.
Typing /afk while in a battleground also makes you leave that battleground
Example:
Person A says: /afk eating food
Person B says: /w A Hello.
Person B recieves: A is Away from Keyboard: eating food'
- Q: What do you call AFK people on a PvP server? A: A Free Kill.
Also see DND.