The Collaborative International Dictionary of English v.0.48:

Cotangent \Co*tan"gent\ (k?-t?n"jent), n. [For co. tangens, an
   abbrev. of L. complementi tangens. See Tangent.] (Trig.)
   The tangent of the complement of an arc or angle. See Illust.
   of Functions.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cotangent
    n 1: ratio of the adjacent to the opposite side of a right-
         angled triangle [syn: cotangent, cotan]