1) Circular function 2) Cotan 3) Precalculus calculation 4) Trigonometric function 1) Cotan 2) Ctn 
• (n.) The tangent of the complement of an arc or angle. See Illust. of Functions.
(from the article `trigonometry`) ...of angles and their application to calculations. There are six functions of an angle commonly used in trigonometry. Their names and abbreviations ...
Co·tan'gent noun [ For co . tangens , an abbrev. of Latin complementi tangens . See Tangent .] (Trig.) The tangent of the complement of an arc or angle. See Illust. of Functions . 
[n] - ratio of the adjacent to the opposite side of a right-angled triangle
Note: In this series A is in radians and See also: Cosecant, Cosine, Secant, Series Expansions, Sine, Tangent.
cotan noun ratio of the adjacent to the opposite side of a right-angled triangle
(in a right triangle) the ratio of the side adjacent to a given angle to the side opposite. · the tangent of the complement, or the reciprocal of the tangent, of a given angle or arc. Abbr.: cot, ctn&hasp;
ratio of the adjacent to the opposite side of a right-angled triangle