According to [Wookieepedia](http://starwars.wikia.com/wiki/All_Terrain_Armored_Transport):

> The name "AT-AT" is never spoken in any of the films. Instead, the machines are referred to as "Imperial walkers."

So, where does the name **AT-AT** originate?

*(Of course, AT-AT stands for **All Terrain Armored Transport**, but neither term is in [the script](http://www.blueharvest.net/scoops/esb-script.shtml).)*