Not on the plan — happening when no timetable said it would. This is the word for the extra stop, the sudden meeting, the landing nobody arranged: an unscheduled stop, an unscheduled visit, an unscheduled press conference. It belongs to the world of timetables and official plans, so you meet it most around planes, trains, hospitals, factories and politics. A plane makes an unscheduled landing; a factory has unscheduled downtime; a president makes an unscheduled trip to the border. Two things to feel. First, it says nothing about whose fault it is or whether it is good or bad — an unscheduled holiday could be lovely, unscheduled maintenance is expensive. It only reports that the plan did not contain this. Second, it carries a faintly official tone, the voice of the announcement rather than the person affected. The airline says "an unscheduled landing"; the passenger says "we had to land early." Almost always used in front of a noun. You would rarely write "the meeting was unscheduled" — more natural is "the meeting wasn't on the schedule."