An Etymological Dictionary of Astronomy and Astrophysics

A group of eight units or figures.
Chemistry: An element, atom, or group having a valence of eight.

See also: From Gk. oktad- (stem oktás) “group of eight,” from okt-→ oct- + -ad a prefix denoting a group or unit comprising a certain number, sometimes of years (e.g. dyad; triad).