Rule 2 – Section 20 – INNINGS
2-20-1 An inning is that portion of the game which includes a turn at bat for each team.
2-20-2 A half-inning is the interval during which one team is on offense (batting) and the other is on defense (fielding). A half-inning ends when there is a third out or when, in the last inning, the winning run is scored. In either case, if there is a delayed out declared by the umpire for a base running infraction, a possible fourth out may be recognized (9-1-1d, e).
2-20-3 An extra inning is one which extends the game in an attempt to break the tie score.