When two or more things coincide, they happen at the same time or match up in some way. If your birthday coincides with your best friend's, you both celebrate on the same day. When spring break coincides with a major soccer tournament, you're lucky because you can attend without missing school.

Things can also coincide when they occupy the same space or align perfectly. In geometry, when two lines coincide, they lie exactly on top of each other, covering the same path. When a detective investigates a crime and discovers that a suspect's story coincides with the facts, the details match up and support each other.

The word often appears when describing lucky or unlucky timing. A family vacation that coincides with a heat wave might be unpleasant, while a school carnival that coincides with perfect weather is fortunate.

A coincidence is when things coincide by accident rather than by planning. If you and your friend both wear the same unusual shirt to school without coordinating, that's a coincidence. Scientists and mathematicians prefer when their theories coincide with experimental results, because that agreement suggests they're on the right track. When paths, schedules, interests, or events line up, they coincide.