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[koh-sahyn] /ˈkoʊ saɪn/
  1. (in a right triangle) the ratio of the side adjacent to a given angle to the hypotenuse.
  2. the sine of the complement of a given angle or arc.
    Abbreviation: cos.
Mathematics. (of a real or complex number x) the function cos x defined by the infinite series 1 − (x 2 /2!) + (x 4 /4!) − + …, where ! denotes factorial.
Abbreviation: cos.
Compare sine (def 3), factorial (def 1).
Origin of cosine
1625-35; < New Latin cosinus. See co-, sine Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for cosine


noun (of an angle)
a trigonometric function that in a right-angled triangle is the ratio of the length of the adjacent side to that of the hypotenuse; the sine of the complement cos
Word Origin
C17: from New Latin cosinus; see co-, sine1
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for cosine

1630s, from co. sinus, abbreviation of Medieval Latin complementi sinus (see complement + sine).

Online Etymology Dictionary, © 2010 Douglas Harper
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cosine in Science
  1. The ratio of the length of the side adjacent to an acute angle of a right triangle to the length of the hypotenuse.

  2. The abscissa of the endpoint of an arc of a unit circle centered at the origin of a Cartesian coordinate system, the arc being of length x and measured counterclockwise from the point (1, 0) if x is positive or clockwise if x is negative.

  3. A function of a number x, equal to the cosine of an angle whose measure in radians is equal to x.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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