The atanh() function in C is used to calculate the inverse hyperbolic tangent (also known as arc hyperbolic tangent) of a given value. It is part of the <math.h> library in C.

The atanh() function takes a single parameter, which represents the value for which you want to calculate the inverse hyperbolic tangent. The parameter must be a floating-point number within the range of -1 to 1 (inclusive) to avoid undefined behavior.