Definition of cosine
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 and that is exactly equal to the cosine of an angle of measure θ in radians : a trigonometric function cos θ that for all real numbers θ is given by the sum of a series of which the first term is 1 and succeeding terms alternate in sign with the kth term consisting of a fraction whose numerator is θ raised to the exponent 2(k  1) and whose denominator is the factorial of 2(k  1) and that is exactly equal to the cosine of an angle of measure θ in radians
Recent Examples of cosine from the Web

The link was so clear it could be described in derivative sines and cosines, mathematical concepts taught in college trigonometry.
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Origin and Etymology of cosine
New Latin cosinus, from co + Medieval Latin sinus sine
COSINE Defined for English Language Learners
cosine
playDefinition of cosine for English Language Learners

geometry : the ratio between the long side (called the hypotenuse) and the side that is next to an acute angle in a right triangle
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Britannica English: Translation of cosine for Arabic speakers Britannica.com: Encyclopedia article about cosine
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