Question: What is a Flash-Sideways?

The term "Flash-Sideways" was first introduced as a result of Episode 6x1, LA X.

Answer:

In the premiere of Lost season 6, we saw our Losties on Flight 815, only things are a little different than they were the first time we saw them on this flight. Co-executive producers Damon Lindelof and Carlton Cuse describe these events as a flash-sideways.

The flash-sideways is a different timeline that appears to be happening parallel to our island timeline, showing what would happen if the plane landed safely in L.A.