Source : Webster's Revised Unabridged Dictionary (1913)

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.