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]

WordNet (r) 3.0 (2006):

trilateral
    adj 1: involving three parties; "trilateral talks"
    2: having three sides; "a trilateral figure" [syn: trilateral,
       triangular, three-sided]
    n 1: a three-sided polygon [syn: triangle, trigon,
         trilateral]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "trilateral":
   bifacial, bilateral, cuneate, cuneated, cuneiform, deltoid,
   dihedral, flanked, handed, lateral, many-sided, multilateral,
   one-sided, oxygonal, polyhedral, quadrilateral, sided, tetrahedral,
   three-cornered, three-dimensional, three-footed, three-pronged,
   three-sided, triangular, triangulate, triarch, trichotomous,
   tricorn, tricornered, tricuspid, trident, tridental, trifid,
   triflorate, triflorous, trifurcate, trigonal, trigonoid,
   trigrammatic, trihedral, trimerous, triparted, tripartite,
   tripodic, triquetral, triquetrous, trisected, two-sided,
   unilateral