It's a known equality that


where z is a complex number, and i the square root of -1.
Now let's assume x is a real (so it's also a complex number). The cool thing about this is that the value of exp(ix) represented in the complex plane will be on a circle with radius of one unit. Also the line going through this point and the point representing 0 will close x degrees (in radians) with the real (the horizontal) axis.

If 0<=x<pi then it's also called the argument of exp(ix).