Definition of "on the ball"

on the ball

prepositional phrase
1. (idiomatic) Alert, active, or attentive; on top of things.
2. (sports) Being in control of the ball.
  Quotations
  ...Rovers were also aided by some poor defending from West Brom, whose lapses at the back undid their excellent work on the ball and condemned Roberto di Matteo's Baggies side to a third straight defeat.
  2010 December 28, Kevin Darlin, “West Brom 1 - 3 Blackburn”, in BBC‎
3. (Australian rules football) Playing as a midfielder.