A triad is a group of three related things that form a meaningful set. In music, a triad is three notes played together to create a chord: the foundation of almost every song you've ever heard. The most basic triad combines the first, third, and fifth notes of a scale, and when you hear a guitar player strum a chord, they're usually playing some kind of triad.

Outside of music, the word describes any important threesome. Teachers might talk about the triad of reading, writing, and arithmetic as the core of elementary education. A story might revolve around a triad of friends whose different personalities create interesting dynamics. Scientists use the term color triad for the three primary colors that combine to make all other colors.

What makes something a triad rather than just three random things is that the three elements work together or relate to each other in a special way. Three strangers standing at a bus stop aren't a triad, but three vertices that form a triangle are.