What is an Avatar?
An avatar, also called a profile picture, is a small image that appears next to your username in forums, online communities, and other social media websites, specially next to your posts and in your profile page.
The term avatar is also abbreviated avvy.
Observations:
Avatars used to very large and prominent in old forums, but nowadays they're very tiny. Old forums also used to have signatures, which gave a lot of personality to each user's posts. Nowadays I can barely tell who is posting what.