The Ising model in two dimensions with {\cal B} = 0 undergoes a phase transition of second order. What it actually means is that below a given critical temperature T_C , the Ising model exhibits a spontaneous magnetization with \langle {\cal M} \rangle\ne 0 . Above T_C the average magnetization is zero. The mean magnetization approaches zero at T_C with an infinite slope. Such a behavior is an example of what are called critical phenomena. A critical phenomenon is normally marked by one or more thermodynamical variables which vanish above a critical point. In our case this is the mean magnetization \langle {\cal M} \rangle\ne 0 . Such a parameter is normally called the order parameter.