block system

Noun

 * 1)  On railways, a system by which the track is divided into sections of three or four miles, and trains are run by the guidance of electric signals so that no train enters a section or block before the preceding train has left it.
 * 2)  Given a group G that acts on a set X, a partition of X that is G-invariant.
 * 1)  Given a group G that acts on a set X, a partition of X that is G-invariant.