A trio is a group of exactly three people or things working together or connected in some way. In music, a trio performs together as a unit: think of three musicians playing a piece written for violin, cello, and piano.

You might read about a trio of friends going on an adventure together, or a trio of scientists making an important discovery. When three things appear together regularly or form a natural set, people call them a trio.

A trio suggests the three parts belong together intentionally, forming a unified group with a shared purpose. Three random students sitting near each other aren't a trio, but three students who always work together on projects might be. A trio has a sense of unity or purpose: three violinists performing together, three buildings designed as a matched set, or three books that tell one continuous story.