The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]