also: プレイボール · プレー・ボール · プレイ・ボール
play ball!
Used as an interjection to start a baseball game or, by extension, any activity.
The umpire shouted, 'Play ball!'
start of a match; play ball
Refers to the moment a baseball, tennis, or similar match begins. Often used in the phrase プレーボールの合図 (signal to start).
The siren signaling the start of the match sounded.