squash ball       
noun [ C ] uk/ˈskwɒʃ ˌbɔːl/ us/ˈskwɑːʃ ˌbɑːl/
 壁球(用球)
a small rubber ball used in the game of squash (= a game played between two or four people on a special closed playing area that involves hitting the ball against a wall)
  In racketball, the ball is bigger and bouncier than a squash ball. 玩英式墙网球时,球比壁球更大、更有弹性。 
 Why does a squash ball bounce higher when it's warm?  为什么壁球在温热的状态会弹得更高? 
 - More examples
 - He is the hardest hitter of a squash ball there has ever been.
 - Squash balls, being made of a rubber compound, are of fairly low resilience.
 - Nothing beats the satisfying, aggressive thwack of a well-hit squash ball.