ETYM Latin trias, -adis, Greek tri- three: cf. French triade. Related to Three, Trias, Trio.
1. A union of three; three objects treated as one; a ternary; a trinity.
2. A chord of three notes; the common chord, consisting of a tone with its third and fifth, with or without the octave.
3. An element or radical whose valence is three.
4. Group of three; Music, chord of three notes.

In music, a chord of three notes consisting of a root, third, and fifth. There are four types of triad: major (for example c–e–g), minor (c–e flat–G), augmented (c–e–G sharp), and diminished (c–e flat–G flat).
Secret society, founded in China as a Buddhist cult ad 36. It became known as the Triad because the triangle played a significant part in the initiation ceremony. Today it is reputed to be involved in organized crime (drugs, gambling, prostitution) among overseas Chinese. Its headquarters are alleged to be in Hong Kong.
In the 18th century the Triad became political, aiming at the overthrow of the Manchu dynasty, and backed the Taiping Rebellion 1851 and Sun Yat-sen's establishment of a republic 1912.