The Collaborative International Dictionary of English v.0.48:

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]