Definition of "interlocking"

In addition to the usual interlocking between conflicting signal and point levers achieved by electric locks, track circuits, when occupied, hold signals governing entry to the relevant line at danger; [...] The up main through inner home signal was held at danger by interlocking, because the track circuits of a conflicting route ahead of it were occupied by another train [...].

1961 March, “Talking of trains: Collision at Waterloo”, in Trains Illustrated, page 138