noun

1. a group of three persons, things, ideas, etc.; trinity
2. a musical chord of three tones, esp. one consisting of a root tone and its third and fifth: a triad with a major third and perfect fifth is called a major triad; a triad with a minor third and perfect fifth is called a minor triad

Etymology: < LL trias (gen. triadis) < Gr trias (gen. triados) < treis, three