unscheduled
adj. B2 Upper Intermediate US //ənˈskɛdʒuɫd// UK //ʌnʃˈɛdjuːld// un·sched·uled
adj. not planned or not on a set time. You use this to describe things that happen by surprise or without being organized in advance.
adj. not planned or arranged in advance; occurring outside of a fixed timetable or calendar. Often used to describe events, meetings, or maintenance that disrupts a routine.
The meeting was unscheduled, so I had to leave my desk.
The airline canceled several flights due to an unscheduled maintenance check on the engines.
While the itinerary was largely structured, the guide left room for unscheduled discoveries, allowing the group to explore local markets that were not on the official map.