Avatar: The Last Airbender (abbreviated as ATLA), also known as Avatar: The Legend of Aang in some regions, is an American anime-influenced animated television series produced by Nickelodeon Animation Studios. It was co-created by Michael Dante DiMartino and Bryan Konietzko, with Aaron Ehasz serving as head writer.
When watching Avatar The Last Airbender, it becomes very apparent that it took major inspiration from the extremely popular—Japanese Anime. Despite it having similar storytelling elements and art style; Avatar The Last Airbender is not an anime.
By American standards, the show isn't an anime since the show was produced in the United States rather than Japan, and the term as we know it is associated with Japanese animation.
Anime refers to a specific style of cartoon produced or inspired by Japanese animation. Think of it this way: all anime shows are cartoons, but not all cartoons are anime. The art style associated with anime is very unique and recognizable.