A jamming transition occurs when the density of particles is increased until a point is reached where the crowding of constituent particles traps them kinetically, thus preventing further exploration of phase space. This is a transition from a liquid-like to a solid-like state, which is characterized by the sudden arrest of dynamics. The jammed solid can be refluidized by thermalization or an applied stress.
Some particles which will undergo jamming transitions include granular media, colloidal suspensions, and molecular systems.
An example of a jamming phase diagram can be found in Jamming phase diagram for attractive particles.
 V. Trappe, V. Prasad, Luca Cipelletti, P.N. Segre, and D. A. Weitz, "Jamming phase diagram for attractive particles," Nature 411 772-775 (2001)