Trilateral \Tri*lat"er*al\, a.[L. trilaterus; tri- (see {Tri-}) + latus, lateris, side: cf. F. trilat['e]ral. See {Lateral}.] (Geom.) Having three sides; being three-sided; as, a trilateral triangle. -- {Tri*lat"er*al*ly}, adv. -- {Tri*lat"er*al*ness}, n.