Tět'ra-hē'dron, n.

n. [Gr. τέτρα, four, and ἕδρα, seat, base, from ἕζομαι, to sit; Fr. tétraèdre.] (Geom.) A solid figure inclosed by four triangles.

Regular tetrahedron (Geom.), a solid bounded by four equal equilateral triangles; one of the five regular solids. Math. Dict.