[ koh-sahyn ]

  1. Trigonometry.

    • (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

  2. 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: cos: Compare sine (def. 3), factorial (def. 1).

Origin of cosine

1625–35; <New Latin cosinus. See co-, sine

Words Nearby cosine Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use cosine in a sentence

British Dictionary definitions for cosine


/ (ˈkəʊˌsaɪn) /

noun(of an angle)
  1. 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: Abbreviation: cos

Origin of 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


[ sīn′ ]

  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.

  1. 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.