How do you spell Triad

Available Definitions:
1)  n. - A union of three; three objects treated as one; a ternary; a trinity; as, a triad of deities.
2)  n. - A chord of three notes.
3)  n. - The common chord, consisting of a tone with its third and fifth, with or without the octave.
4)  n. - An element or radical whose valence is three.