triad
Cross references: 1. common chord tri.ad \'tri--.ad, -*d\ \tri--'ad-ik\ \-i-k(*-)le-\ n [L triad-, trias, fr. Gk, fr. treis three] 1: a union or group of three esp. of three closely related persons, beings, or things : TRINITY 2: a chord of three tones consisting of a root with its third and fifth and constituting the harmonic basis of tonal music - tri.ad.ic aj