a. and sb. [f. L. trilater-us three-sided + -AL. Cf. F. trilatéral.]
A. adj. Contained by three sides; three-sided.
1660. Barrow, Euclid, I. Def. xx. Three sided or Trilateral figures are such as are contained under three right lines.
1788. T. Taylor, Proclus, I. 173. Euclid appears to me to have made a separate division into angles and sides, from considering this alone, that every triangle is not also trilateral.
1828. Hutton’s Course Math., II. 136. The quadrilateral space EAA′E′ is double the trilateral space AA′F.
1875. Merivale, Gen. Hist. Rome, xviii. (1877), 102. Two powers now remained to struggle for the dominion of the trilateral island [Sicily].
B. sb. A three-sided figure; a triangle.
1766. Compl. Farmer, s.v. Surveying, Triangles are figures comprehended under three right lines, and … might be better called trilaterals.
1870. Observer, 9 Oct. From the canal, round the trilateral of St. Denis, to the Seine about Argenteuil, the Prussian Guards … have their stations.
1885. Leudesdorf, Cremona’s Proj. Geom., 31. It follows that the triangles (trilaterals) bcd, b′c′d′ are also in perspective.
Hence Trilaterality, Trilateralness, the quality of being trilateral; Trilaterally adv., in a trilateral form, triangularly.
1837–8. Sir W. Hamilton, Logic, xi. (1866), I. 208. [A] triangle [is distinguished] from every other class of mathematical figures, by the single character of *trilaterality.
1847. Webster, *Trilaterally.
1727. Bailey, vol. II., *Trilateralness, the having three sides.
