“ATL” is one of the commonest acronyms used on AH.com and its discussion board. It simply means “Alternate timeline” - i.e., any alternate history, any allohistorical scenario, timeline or story. Every ATL represents a fictional, parallel universe to OTL, with alternate history properties. In essence, every single alternate history ever written or created through other means is an ATL.
OTL - The acronym for the concept of our own timeline, our own historical universe, in the genre of alternate history.