A triplet is one of three children born to the same mother in the same birth. When a woman gives birth to three babies during the same pregnancy, those babies are triplets. They share a birthday and grow up together from the very start.

Triplets can be identical (meaning they all developed from the same fertilized egg and look extremely similar) or fraternal (meaning they developed from different fertilized eggs and might look as different as regular siblings). Having triplets is rare: only about 1 in every 10,000 births results in triplets.

In music, a triplet means three notes played in the time normally taken by two notes of the same value. If you're tapping a steady beat and suddenly fit three quick taps into one beat, you've played a triplet. Musicians mark triplets with a small “3” above or below the notes.

The word can also describe any set of three similar things. A triplet of stars means three stars close together in the night sky. Scientists might refer to a triplet of experiments when they run the same test three times to make sure their results are reliable.