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1.
A flash sideways is not a flashback, flash-forward or an alternate timeline. It posits what would have happened if one event didn't.
In the ABC Television show Lost, a flash sideways shows what would have happened if Oceanic flight 815 didn't crash on the island but instead landed in Los Angeles.
by babcock69 May 23, 2010
 
2.
Similar to a flashback or flashforward but shows events that would transpire if a character's life would have turned out differently than what happened in the original timeline.

This concept was first started in the 6th season of the tv series Lost.
Guy1: "Man, imagine what my life would be like in a flashsideways timeline if Emily didn't break up with me?"

Guy2: "I know man, I know. Just let the tears out!"
by MovieFreak1_O.o February 17, 2010