  Hysteresis is observed at second order phase transitions. Universal scaling
formul\ae{} for the areas of hysteresis loops are written down. Critical
exponents are defined, and related to other exponents for static and dynamic
critical phenomena. These relations are verified with Langevin dynamics in both
the critical and tricritical mean-field models. A finite-size scaling relation
is tested in the two-dimensional Ising model with heat-bath dynamics. (Shar
file; 2 figures created through dvips directives from 4 PS files included)
