(in a right triangle) the ratio of the side adjacent to a given angle to the hypotenuse.
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 ).

1625–35; < Neo-Latin cosinus. See co-, sine1 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
cosine (ˈkəʊˌsaɪn)
cos 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
[C17: from New Latin cosinus; see co-, sine1]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1630s, from co- + sine.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
cosine   (kō'sīn')  Pronunciation Key 
  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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Computing Dictionary

COSINE definition

Cooperation for Open Systems Interconnection Networking in Europe. A EUREKA project.

The Free On-line Dictionary of Computing, © Denis Howe 2010
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Example sentences
From a mathematical point of view, correlation is the cosine of an angle
  between two data vectors.
Two functions that meet those requirements are the sine and cosine functions.
The cosine and sine functions cause periodic nulls, unless they are cancelled
  out by the term in the denominator.
Cosine response characteristics of radiometric and photometric sensors.
Image for cosine
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