An event or situation coincides when two or more things happen at the same time.
You often hear "coincide" when talking about strange or unexpected connections between events. For example, if someone says their flight was delayed and they arrived just as a major storm hit, it's because the plane coincided with the bad weather. In other cases, people might use coincide to describe two events that happened at exactly the same moment in history - like a famous speech given on July 4th and another event occurring on the same day somewhere else in the world.
To occupy exactly the same space.
Example: The two squares coincide nicely.
