cis is often used by mathematicians
as a shorthand as
cis(x) = cos(x) + i*sin(x)
This function is often used when speaking of vectors in a 2-dimensional plane representing the set of complex numbers. In this use (keeping with the example above) x represents the angle a unit vector makes with the positive real axis and cis(x) its complex representation.
Perhaps one of its most famous uses is in Euler's formula, relating the exponentiation function to the trigonometric functions.