Etymology
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cosine (n.)

one of the three fundamental functions of trigonometry,  1630s, contraction of co. sinus, abbreviation of Medieval Latin complementi sinus (see complement + sine). The word was used in Latin c. 1620 by English mathematician Edmund Gunter.

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Definitions of cosine

cosine (n.)
ratio of the adjacent side to the hypotenuse of a right-angled triangle;
Synonyms: cos
From wordnet.princeton.edu