An avatar is a computer user's representation of himself/herself or alter ego, whether in the form of a three-dimensional model used in computer games, a two-dimensional icon (picture) used on Internet forums and other communities, or a text construct found on early systems such as MUDs.
It is an "object" representing the embodiment of the user. The term "avatar" can also refer to the personality connected with the screen name, or handle, of an Internet user.
The use of Avatar to mean online virtual bodies was popularised by Neal Stephenson in his cyberpunk novel Snow Crash (1992). In Snow Crash, the term Avatar was used to describe the virtual simulation of the human form in the Metaverse, a virtual-reality version of the Internet.
Social status within the Metaverse was often based on the quality of a user's avatar, as a highly detailed avatar showed that the user was a skilled hacker and programmer while the less talented would buy off-the-shelf models in the same manner a beginner would today.
Find out more about Avatar on Wikipedia.