Pertaining to a trigon; having the form of a trigon; triangular.In entomology, triangular in cross-section; having three long edges; trihedral; prismatic: as, trigonal antennæ; trigonal joints.In botany, same as trigonous.In anatomy, noting a triangular space at the base of the bladder. See trigonum .n. In anatomy, the triangular space at the base of the bladder; the trigonum.