[: More must be said here about the complex functions' ranges.]

#include <math.h>
float coshf(float a);
double cosh(double a);
long double coshl(long double a);

#include <complex.h>
float complex ccoshf(float complex a);
double complex ccosh(double complex a);
long double complex ccoshl(long double complex a);

#include <tgmath.h>
floating-type cosh(floating-type a);

The cosh functions take a hyperbolic angle a and return the hyperbolic cosine of that angle, cosh a, in the range [+1, +∞].

#include <math.h>
float acoshf(float r);
double acosh(double r);
long double acoshl(long double r);

#include <complex.h>
float complex cacoshf(float complex r);
double complex cacosh(double complex r);
long double complex cacoshl(long double complex r);

#include <tgmath.h>
floating-type acosh(floating-type r);

The acosh functions take a ratio r and return the area hyperbolic cosine of that ratio, cosh⁻¹ r.