Ball games are games that are played with a ball such as tennis, baseball, and football
You can use ball game to describe any situation or activity, especially one that involves competition. Two of his biggest competitors are out of the ball game. If you say that a situation is a new ball game, you mean that it is completely different from, or much more difficult than, the previous situation or any situation that you have experienced before. He finds himself faced with a whole new ball game
A completely different situation, often one which is difficult or which you know little about. "We'd done a lot of climbing in Scotland but the Himalayas were a whole new ball game."
[ 'bol ] (noun.) 13th century. Middle English bal, from Old Norse bollr; akin to Old English bealluc testis, Old High German balla ball, Old English blAwan to blow.