tetrahedral adjective. [tɛtrǝ'hi:dr(ǝ)l, -'hɛdr(ǝ)l] L18.
[from late Greek tetraedros (see TETRAHEDRON) + -AL¹. Cf. -HEDRAL.]
1. Of a prism, pyramid, etc.: having four sides or faces (in addition to the base or ends); bounded by four plane surfaces. L18.

2. Of, pertaining to, or having the form of a tetrahedron; having axes of symmetry corresponding to the vertices of a tetrahedron; spec. in Crystallography, belonging to a division of the cubic system in which the regular tetrahedron is the characteristic form. E19.
Special collocations: tetrahedral number each of the series of integers 1, 4, 10, 20,..., the nth member of which is the sum of the first n triangular numbers.

■ tetrahedrally adverb in a tetrahedral manner or form M19.

■ tetrahedric adjective tetrahedral: only in L19.

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