[n] the cardinal number that is the sum of three and one
4, four, foursome, IV, Little Joe, quadruplet, quartet, quatern, quaternary, quaternion, quaternity
\Tet"rad\, n. [L. tetras, -adis, Gr. ?, ?: cf. F. t['e]trade.]
1. The number four; a collection of four things; a quaternion.
2. (Chem.) A tetravalent or quadrivalent atom or radical; as, carbon is a tetrad.