Actually, I did know, although I may of momentarily forgotten.
In a Twitter exchange, Anil Dash just reminded me that the word “avatar” comes from from the Sanskrit word Avatãra. The word means, more or less, “descent.” More, from a related blog post at Heritage Key:
But while the modern day meaning implies gaming and interaction, the original definition has a very different meaning. In Hinduism, avatars act as manifestations of deities. This occurs when a god has decided to come to our world by taking a human or animal form.The most well-known avatars were associated with the god Vishnu, who often appeared in our world to restore good in the world when evil threatened to corrupt it. The deity would do so by fighting off demons as a fish or a boar. At other times, Vishnu would lead armies to victory as an eventual king (Sounds a little similar to the plot of the movie Avatar?).