The Collaborative International Dictionary of English v.0.48:
Trigonid \Tri"go*nid\ [Webster 1913 Suppl.] Trigonal \Trig"o*nal\, a. Having three angles, or corners; triangular; as, a trigonal stem, one having tree prominent longitudinal angles. [1913 Webster] [1913 Webster]WordNet (r) 3.0 (2006):
trigonal adj 1: having threefold symmetry [syn: rhombohedral, trigonal]