Regulation Game

What does Regulation Game Mean?

In baseball, the portion of a game that spans nine innings of play, or a game that has been ruled complete due to a rainout or by official rule. A regulation game will also run for less than nine innings if the home team scores the winning run at any time during the bottom of the ninth.

Sporting Charts explains Regulation Game

When a game is tied at the end of nine innings it continues, but it no longer considered regulation. At this point the game is considered to be in "extra innings", and will conclude at the end of the bottom half of any subsequent inning when the visiting team is in the lead, or immediately upon the home team scoring the go-ahead run.                          

There is also an optional rule in Major League baseball that allows the league to shorten one or both of the games of a doubleheader to seven innings, with the same rules for the ninth inning applying to the seventh.