The time required for a star or galaxy to move from one side of a gravitationally bound cluster to the other, if it is travelling at the typical random speed of objects in the cluster. This is the shortest time-scale on which significant dynamical events can occur, such as changes in the shape of the cluster and exchange of energy between cluster members. For a globular cluster the crossing time is approximately 100 000 years; for a rich cluster of galaxies it is about a billion years.