The trigonometric cosine function.
xis a real or complex number
x is a real number,
cos(x) is the
x coordinate of the intersection of the unit circle
x^2 + y^2 = 1 and the ray from the origin making an angle
x (in radians) from the positive
x axis in the anticlockwise direction.
z is a complex number,
cos(z) is defined as
[e^(i⋅z) + e^(−i⋅z)]/2.
If you want to specify the argument in degrees, you can use the ° postfix operator, which amounts to multiplication by π/180.