A "coincidence" is when something that seems unlikely happens. Here are some examples of coincidences:

• You're vacationing in another country, and you run into one of your friends from college who's on a vacation there as well.
• You check your account balance at an ATM, and the amount of money in your account is exactly equal to the year that you were born.
• Three movies come out in the same season which are all based on the same children's story.

When something coincidental happens, you can say:

What a coinclidence!