Imagine, I can still remember the definition from trigonometry.
cotangent
n.
5ModL cotangens < co. tangens, short for complementi tangens, lit., tangent of the complement6 Trigonometry the reciprocal of the tangent; specif. a) the ratio of the adjacent side of a given acute angle in a right triangle to the opposite side (see SINE, illus.) b) an equivalent, positive or negative ratio for certain related angles (Ex.: the cotangent of 57U or 237U is .6494, of 123U or 303U is /.6494) or real numbers representing radians (Ex.: the cotangent of .9948 radians (57U) is .6494)