- the ratio of the adjacent side of a given acute angle in a right triangle to the hypotenuse
- an equivalent, positive or negative ratio for certain related angles (Ex.: the cosine of 57° or 303° is .5446, of 123° or 237° is ?.5446) or real numbers representing radians (Ex.: cosine of .9948 radians (57°) is .5446)

Origin of cosine

Modern Latin from*co. sinus,*short for

*complementi sinus,*literally , sine of the complement