The flock centring rule of a flocking system (also known as cohesion) makes flock members attempt to get as near to the centre of the flock as possible.

The centre of the flock is a relative concept, based on the relative position of each flock member to its nearest neighbours. If the flock member has neighbours on all sides, its flock centring urge is small because it is already near to its flock centre. However if most of its neighbours are to one side, then it has a greater urge to get to the centre of the flock, and so attempts to steer towards them.