In a way, no. Avatar wasn't made in Japan, so in countries other than Jepun it is not considered an anime. But in Japan, "anime" refers to any form of animasi regardless of nationality. Therefore, in Japan, it is considered an anime. It is not considered an Anime in other countries such as the United States.
It's sort of a confusing choice between cartoon and anime, since Anime refers to animasi made in Jepun and Avatar was made in America it would make lebih sense for it to just be a cartoon.
Well, "anime" is the Japanese word for Kartun and animation, so, in a way, yes, but Anime really refers to the Japanese made cartoons, so in another way--no.