An appeal by the crowd for holding the ball against a tackled player. This is heard almost any time an opposition player is tackled, without regard to whether the rules about "prior opportunity" to dispose of the ball are fulfilled
2007: A good tackle (and some bad ones) will bring a cry of Ball! from the crowd – a plea for a holding the ball free kick. — AFL Sydney Swans Rules Zone.
If someone is on the ball, they are very alert and aware of what is happening. She really is on the ball; she's bought houses at auctions so she knows what she's doing
The set of points in a metric space lying within a given distance (the radius) of a given point; specifically, the homologue of the disk in a Euclidean space of any number of dimensions