In short and simple terms, an "alternate timeline" refers to a different version of events or history that could have happened if certain things had changed. It suggests a scenario where things turned out differently from what actually occurred.
Sentences with «alternate timeline»
Or would you go sideways in time to see alternate timelines where the world is shifted into a new reality? (storybundle.com)
As fun as imagining alternate timelines is, I wouldn't have it any other way at this point. (miketendo64.com)
Will he succeed and create a new alternate timeline? (cbr.com)