\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.

2. A treatise in this science.

{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.