cosine

noun

co·​sine ˈkō-ˌsīn 

1

: a trigonometric function that for an acute angle is the ratio between the leg adjacent to the angle when it is considered part of a right triangle and the hypotenuse

2

: a trigonometric function cos θ that for all real numbers θ is given by the sum of the alternating series {latex}1 - \frac{x^2}{2!} + \frac{x^4}{4!} - \frac{x^6}{6!} + \frac{x^8}{8!} - \dots{/latex} and that is exactly equal to the cosine of an angle of measure θ in radians

Examples of cosine in a Sentence

Recent Examples on the Web

According to the latest bound, proved in 2004 by Imre Ruzsa of the Alfréd Rényi Institute of Mathematics in Hungary, a sum of 1020 cosines — that’s a 1 with 20 zeros after it, about the number of molecules in a cubic inch of air — must have a minimum value smaller than about −7. — Leila Sloman, Quanta Magazine, 28 Jan. 2026

The stronger the cosine transformation, the more compressed the final result. — IEEE Spectrum, 17 June 2025

In the case of something basic, such as the sine or cosine functions used in trigonometry, that geometry is a simple circle. — Ramin Skibba, Scientific American, 13 May 2025

These approaches rely on mathematical functions like squaring, adding, or multiplying, rather than irrational numbers, radicals, or functions like sine and cosine. — Jack Beresford, MSNBC Newsweek, 1 May 2025

Word History

Etymology

New Latin cosinus, from co- + Medieval Latin sinus sine

First Known Use

1635, in the meaning defined at sense 1

Phrases Containing cosine

• arc cosine
• direction cosine

Rhymes for cosine

• beanie
• genie
• sweeney
• sweeny
• teeny
• acini
• aldine
• amine
• arsine
• carinii
• delphine
• domine

Kids Definition

cosine

noun

co·​sine ˈkō-ˌsīn 

: a trigonometric function that is the ratio between the side next to an acute angle in a right triangle and the hypotenuse