The arc-cosine is the inverse of the cosine function. Calcute implements it with the acos function. The inverse cosine of a value x is the value y for which the cosine of y is x. In other words if y = acos(x) then x = cos(y). And since the cosine takes an angle as input parameter, the arccosine produces an angle as its output.
The acos value is expressed by Calcute using the currently-selected angle unit: radian, degree or gradient. In the above example, the result is in radians.
The arc-cosine function is only defined for parameter values in the close range
In some mathematical texts and hand-held calculators, the notation cos-1 is sometimes used to represent the arc-cosine function. This is an unfortunate notational choice since it could also stand for the multiplicative inverse 1/cos, which is a different function.
Yet another term also used to represent the inverse cosine function is "arccos".