cis
The expression cos x + i sin x is abbreviated with cis x and is therefore only another name for Euler's formula
Explanation
Trigonometric functions can be defined with complex numbers.
For each real number x, Euler's formula holds in the form
Here e is the base of the natural logarithm, i is the imaginary unit, and in the sine and cosine x are expressed in radians. The general form of the complex exponential function is