Wow, I guess I'm not a Star Wars fan after all. I had no idea.....
You can be a fan and not know what the other name for Imperial Walkers was. I do not think the terms, "ATAT" or "AT-AT" were ever used in the movies. They always called them Imperial Walkers. I learned the term AT-AT when I was a kid reading Star Wars stuff.. like Comic Books, coloring books, figurine packages etc. I had a AT-AT model at one point too.