An Avatar is an alternate consciousness linked to another, usually that of a Deity.
The Deities of the Pantheon have always had the ability to create offshoots of themselves in order to work inside the physical universe without altering it too much, as would be the case if their complete essences were translated onto the physical plane. Avatars were the first version of these offshoots, and are still commonly used.
An Avatar's reason for existence is as a vessel for a fragment of a Deity's consciousness. Though Avatars are usually created with a specific purpose or end result in mind, this is not always the case; some are made simply to allow the Deity a method of connection with the physical world as a temporary change of pace.
Even if a specific objective is achieved, the Avatar may continue to exist for a significant amount of time, unlike a Shard.
Deities are always directly aware of their Avatars at some level of consciousness, due to the link that exists between them. Most Deities prefer to only use a single Avatar at a time, or perhaps a handful, but this is not always the case and there is no clearly established limit.
Avatars serve as conduits for the power of their respective Deities, allowing them to partially bypass the magic restriction by the same mechanic that priests use, namely, having a separate focus for the Deities' power.