Unplanned means happening without preparation or intention, like something that occurs spontaneously rather than through careful organization. When your family takes an unplanned detour during a road trip because you spot an interesting roadside attraction, you're making a decision in the moment rather than following your original route.

Unplanned can describe events that simply weren't anticipated. An unplanned snow day means no one knew school would be canceled until that morning. An unplanned expense, like a bike repair after you hit a pothole, forces you to spend money you hadn't budgeted for.

Sometimes unplanned things turn out wonderfully: an unplanned conversation with a new classmate might spark a great friendship. Other times they create challenges: an unplanned power outage during your presentation means scrambling for a backup plan. The word itself is neutral, simply describing the absence of advance planning, though whether that's good or bad depends entirely on the situation.