Name sometimes given to certain real functions such as trigonometric functions to relate them to one another.
Example
Because sin(a) = cos(\(\frac{π}{2}\) – a), the sine function is the cofunction of the cosine function, and vice versa.
Because sin(a) = cos(\(\frac{π}{2}\) – a), the sine function is the cofunction of the cosine function, and vice versa.