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.

Source : WordNet®

trilateral
     adj 1: involving three parties or elements; "the triangular
            mother-father-child relationship"; "a trilateral
            agreement"; "a tripartite treaty"; "a tripartite
            division"; "a three-way playoff" [syn: {triangular}, {tripartite},
             {three-party}, {three-way}]
     2: having three sides; "a trilateral figure" [syn: {triangular},
         {three-sided}]
     n : a three-sided polygon [syn: {triangle}, {trigon}]