trigonal adjective. ['trɪg(ǝ)n(ǝ)l] L16.
[medieval Latin trigonalis triangular, from Latin trigonum triangle: see TRIGON, -AL¹.]
1. a. Of or pertaining to a triangle; triangular. L16.
b. Geometry. Having triangular faces; having the symmetry of an equilateral triangle; spec. (Crystallography) of, pertaining to, or designating a crystal system referred to three equal axes separated by equal angles not right angles. L19.
trigonal bipyramid a solid figure having the form of two triangular pyramids joined base to base. trigonal quoin a solid angle contained by three plane angles. trigonal symmetry the symmetry of a figure or body which coincides with its original position after rotation about an axis through an angle of 120 or 240.
c. Chemistry. Having three orbitals directed to the corners of an equilateral triangle. M20.

2. Triangular in cross-section. L16.

3. Astrology. Pertaining to or of the nature of a trigon. E-M17.

■ trigonally adverb L19.

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