Undecided. Something remains unresolved. A decision or outcome has not been made or has not been determined. You may be up in the air about what to eat tonight or where to perform your wedding, anytime a decision has yet to be made. ‘Up in the air’ illustrates something floating above that has not yet landed to become firm. (Compare to Jury is out or On the fence. Contrast with Nail down.)
“Our vacation plans are still up in the air”.