Avatar - A virtual representation of a user. This representation can come in many forms, whether human, animal, object, etc. Essentially, an avatar is someone's character they use to represent themselves in virtual reality. An avatar could be a screen name, blog name, user name, videogame character, etc. An avatar can also refer to the 'icon' or picture that identifies a specific user in addition to that user as an entity.
Link to 'What's New, New Media' wiki's entry on avatars: Avatar