In many materials, the Curie–Weiss law fails to describe the susceptibility in the immediate vicinity of the Curie point, since it is based on a mean-field approximation. Instead, there is a critical behavior of the form
with the critical exponent . However, at temperatures  the expression of the Curie–Weiss law still holds true, but with  replaced by a temperature that is somewhat higher than the actual Curie temperature. Some authors call  the Weiss constant to distinguish it from the temperature of the actual Curie point.