Trigonometry \Trig`o*nom"e*try\, n.; pl. {-tries}. [Gr. ? a triangle + -metry: cf. F. trigonom['e]trie. See {Trigon}.] 1. That branch of mathematics which treats of the relations of the sides and angles of triangles, which the methods of deducing from certain given parts other required parts, and also of the general relations which exist between the trigonometrical functions of arcs or angles. [1913 Webster] 2. A treatise in this science. [1913 Webster] {Analytical trigonometry}, that branch of trigonometry which treats of the relations and properties of the trigonometrical functions. {Plane trigonometry}, and {Spherical trigonometry}, those branches of trigonometry in which its principles are applied to plane triangles and spherical triangles respectively. [1913 Webster]