A game is considered a regulation game -- also known as an "official game" -- once the visiting team has made 15 outs (five innings) and the home team is leading, or once the home team has made 15 outs regardless of score.
Prior to the 2020 season, if a game was terminated early due to weather before becoming official, the results up to the point of the termination did not count and the game was started over at a later date. But as part of MLB's health and safety protocols during the COVID-19 pandemic, all games cut short due to weather before becoming official were resumed at a later date, rather than started over from scratch, during the 2020 campaign.
The rules below remained in place.
If a regulation game is terminated early due to weather, the results are considered final if the home team is leading. If the home team is trailing, the results are considered final if the game is not in the midst of an inning when the visiting team has taken the lead.
If a regulation game is terminated early due to weather and the game is either tied or in the midst of an inning in which the visiting team has taken the lead, it becomes a suspended game that will be completed at a later date from the point of termination.
If not terminated early, regulation games last until the trailing team has had the chance to make 27 outs (nine innings). If the home team is leading after the visiting team has made three outs in the top of the ninth inning, the home team wins and does not have to come to bat in the bottom of the ninth.
If the game is tied after both teams have made 27 outs each, the game will go to extra innings. It will continue until the home team takes the lead at any point, or the visiting team takes the lead and the home team subsequently makes three outs without tying the game or going ahead.