  Collective cell motility is crucial to many biological processes including
morphogenesis, wound healing, and cancer invasion. Recently, the biology and
biophysics communities have begun to use the term cell jamming to describe the
collective arrest of cell motion in tissues. Although this term is widely used,
the underlying mechanisms are varied. In this review, we highlight three
independent mechanisms that can potentially drive arrest of cell motion --
crowding, tension-driven rigidity, and reduction of fluctuations -- and propose
a speculative phase diagram that includes all three. Since multiple mechanisms
may be operating simultaneously, this emphasizes that experiments should strive
to identify which mechanism dominates in a given situation. We also discuss how
specific cell-scale and molecular-scale biological processes, such as cell-cell
and cell-substrate interactions, control aspects of these underlying physical
mechanisms.
