[ 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 − (x2/2!) + (x4/4!) − + …, where ! denotes factorial. Abbreviation: cosCompare sine (def. 3), factorial (def. 1).



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Origin of cosine

1625–35; <New Latin cosinus. See co-, sine Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

Example sentences from the Web for cosine

British Dictionary definitions for cosine

/ (ˈkəʊˌsaɪn) /

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 complementAbbreviation: cos

Word Origin for cosine

C17: from New Latin cosinus; see co-, sine 1
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

Scientific definitions for cosine

[ kōsīn′ ]

The ratio of the length of the side adjacent to an acute angle of a right triangle to the length of the hypotenuse.
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.
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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.