Unforeseen means not predicted or expected beforehand. When something is unforeseen, it catches you by surprise because you had no way of knowing it would happen.

A science fair might get canceled due to unforeseen circumstances like a sudden snowstorm or a power outage. A chess player might lose a match because of an unforeseen move from their opponent, something they never anticipated in all their planning. When a school play has to deal with unforeseen problems, it might be a missing costume, a broken prop, or an actor getting sick.

Something foreseen is predictable, like knowing you'll need to study for next week's spelling test. Something unforeseen is unpredictable, like a fire drill interrupting the test.

Unforeseen events aren't necessarily bad. You might have an unforeseen chance to visit your grandparents when they surprise you with a weekend trip. A scientist might make an unforeseen discovery while researching something completely different. The word simply means that whatever happened, nobody saw it coming. Good planning helps you prepare for likely problems, but unforeseen events remind us that we can't predict everything.