“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 out into one of the countless alternate timelines (ATL).
Since “Fact is stranger than fiction” and real history alone has many peculiar events, the board often features jokes about how OTL is the most implausible timeline ever.
Small OTL History Questions thread (Before 1900) - Need advice on smaller but important aspects of OTL history ? Try this thread.
ATL - The acronym for the concept of an alternate timeline and/or alternate universe in the genre of alternate history.