1. (a.) Having, or composed of, four sides.

2. (a.) Having the form of the regular tetrahedron.

3. (a.) Pertaining or related to a tetrahedron, or to the system of hemihedral forms to which the tetrahedron belongs.

Janus-like ambidextrous bifacial bifold biform bifurcated binary binate biparous bipartisan bipartite bivalent conduplicate dichotomous dihedral disomatous double double-faced duadic dual dualistic duple duplex duplicate duplicated dyadic flanked geminate geminated handed identical lateral many-sided matched multilateral one-sided polyhedral quadrilateral second secondary sided tetrahedral three-sided trihedral trilateral triquetrous twain twin twinned two two-faced two-level two-ply two-sided two-story twofold unilateral