a group of three, esp. of three closely related persons or things.

Chemistry .

an element, atom, or group having a valence of three. Compare monad ( def. 2 ) , dyad ( def. 3 ) .

a group of three closely related compounds or elements, as isomers or halides.

Music . a chord of three tones, esp. one consisting of a given tone with its major or minor third and its perfect, augmented, or diminished fifth.

( initial capital letter ) Military . the three categories of strategic-nuclear-weapons delivery systems

bombers, land-based missiles, and missile-firing submarines.

noun

three , threesome , trine , trinity , triple , triumvirate , triune , triunity , troika