"Extra inning" refers to a situation in baseball when the normal number of innings (usually nine) is not enough to determine a winner, so additional innings are played to break the tie. Full definition
There's still a lot of regular baseball going on in the game, though, which in the 2017 edition, meant we ended up with a 1 - 1 game going into extra innings in a game that doesn't count for anything that didn't see its first pitch until 8:22 p.m. ET. (sbnation.com)
There are too many ball clubs; the playoffs require playing in 40 degree weather; and in the case of last night, the Yankees were not allowed a day off after playing extra inning games with Baltimore. (nystateofpolitics.com)
Last season, Ben Zobrist took home the hardware after driving in the game winning run in extra innings of game 7. (sportsinsights.com)