In this image of Saturn, the Cassini Division is the gap between the two widest rings

The Cassini Division is a 4,800 km (2,980 mile) gap between Saturn's A Ring and B Ring. It was discovered in 1675 by Giovanni Cassini.

All of the gaps in Saturn's rings are caused by the gravitational pull of one or more of Saturn's moons affecting the orbits of the tiny particles of which the rings are comprised. Saturn's moon Mimas is responsible for clearing the material from the Cassini Division.