One of a wide variety of materials that contain interacting atomic magnetic moments and also possess some form of disorder, in which the temperature variation of the magnetic susceptibility undergoes an abrupt change in slope, that is, a cusp, at a temperature generally referred to as the freezing temperature, T_f. At temperatures below T_f the spins have no long-range magnetic order, but instead are found to have static or quasistatic orientations which vary randomly over macroscopic distances. The latter state is referred to as spin-glass magnetic order. Below T_f these systems also exhibit magnetic hysteresis and slow relaxation phenomena. Analogous effects are observed in measurements of torque in an applied magnetic field and in electron paramagnetic resonance.