Abbreviation for the TV show Avatar: The Last Airbender. Also written ATLA or AtLA. An incorrect and unacceptable way of abbreviating the title would be ATLAB. The B is added by the ignoramuses of the fandom who don't realize that 'Airbender' is ONE WORD.
Writing A:TLA instead of the full title saves a lot of time when texting you're friends to alert them that the show is starting.