Alternate history is a subgenre of historical fiction (and sometimes of science fiction and/or fantasy). It involves the changing of a certain historical event to produce a different outcome and traces the consequences of that alteration through historical events effected by it, comparing and contrasting them to their OTL equivalents. This altered history is often called an alternate timeline, ATL for short.
Many of Harry Turtledove's best known and most popular works fall into this category. The Southern Victory series is an example of straight-forward historical alternate history where the Point of Divergence is a historical event (the recovery of Special Orders 191) that didn't happen in OTL. The Crosstime Traffic series is an example of the "many worlds" hypothesis of physics with the ability of a society in the "home timeline" to travel across timelines to "alternates" of differing history. The Guns of the South is a science fictional alternate history involving time travel. The Worldwar franchise is a science fictional alternate history involving an alien invasion of the Earth during World War II.