D defs.my
Entry 1 sense Webster, 1913

Tetrahedron

/tĕtrəhē'drən/ · Tet·ra·he·dron · IPA /ˌtɛtɹəˈhiːdɹən/
01 n. A solid figure inclosed or bounded by four triangles.
  1. 1.
    A solid figure inclosed or bounded by four triangles.(Geom.)
Phrases & compounds
Regular tetrahedron — a solid bounded by four equal equilateral triangles; one of the five regular solids.