“OTL” is one of the commonest acronyms used on AH.com and its discussion board. It simply means “Our timeline” - i.e., the history of the world as we know it, everything but the history of a fictional, parallel universe with alternate history properties. In essence, we live in OTL. If we'd ever change the past of OTL, it would automatically stop being OTL and branch into one of the countless alternate timelines.

AH.com's AH Writing Slang

ATL - The acronym for the concept of an alternate timeline and/or alternate universe in the genre of alternate history.

ASB - The acronym for the concept of Alien Space Bats in the genre of alternate history.

ISOT - The acronym for personal or mass teleportation to other historical eras or other worlds/setting, usualy initiated via unexplained ASB means.