Arccos, also known as the inverse cosine function, is the inverse function of the cosine function. It is denoted as arccos or cos^-1 and is used to find the angle whose cosine value is given.

The arccos function takes a value between -1 and 1 as its argument and returns an angle value in radians between 0 and pi (or between 0 and 180 degrees).

The formula for arccos is:

arccos(x) = cos^-1(x) = y

where x is the input value, y is the angle whose cosine is x, and y is in radians.

For example, arccos(0.5) = cos^-1(0.5) = 1.047 radians (or approximately 60 degrees). This means that the cosine of 1.047 radians (or 60 degrees) is 0.5.