AAT is an initialism for

1. Aquatic Ape Theory, a theory that states that the very early hominids had a semi-aquatic phase in which they lived in a littoral area and fed largely on fish and shellfish.
2. The Anglo-Australian Telescope
3. AAT was an Aircraft manufacturer
4. Apple Advanced Typography, a text imaging technology from Apple Computer.
5. The Armoured Assault Tank from Star Wars, used by the Trade Federation