First Known Use: 1635
Dictionary
cosine
noun co·sine \ˈkō-ˌsīn\
geometry : the ratio between the long side (called the hypotenuse) and the side that is next to an acute angle in a right triangle
Full 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
and that is exactly equal to the cosine of an angle of measure θ in radians See cosine defined for English-language learners
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Origin of COSINE
New Latin cosinus, from co- + Medieval Latin sinus sine
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