Inning
The unit a baseball game is measured in, and the reason baseball has no clock.
What it actually is. An inning is one turn at bat for each team. The visiting team hits first, which is the top of the inning, and once they make three outs the home team hits, which is the bottom. When both halves are done the inning is over. A full game is nine of them, and a game does not end until they are finished, which is why baseball has no clock and no way to run one out.
What somebody who knows the game gets out of it. The innings structure is what makes baseball a game of possessions rather than of time, and almost every strategic idea in the sport comes out of that. You cannot sit on a lead, so a three run cushion in the ninth is a real thing rather than a matter of holding the ball. You cannot hurry, so the losing team always gets its full turn. And because outs are the currency instead of seconds, giving one away is a genuine cost that can be measured, which is where run expectancy comes from and why bunting fell out of fashion.
The numbers worth knowing. Nine innings, twenty seven outs a side. A starting pitcher going six is a quality outing, seven is a good night and nine is rare enough to make the highlights. The home team does not bat in the bottom of the ninth if it is already ahead, which is why home teams play fewer half innings across a season than road teams do.
What people get wrong about it. The bottom of the ninth is not guaranteed. A home team leading after the top of the ninth has won, so the game ends there. It is the one place in sport where the winning side deliberately does not take its turn, and it is also why a home team's win is either a walk off or a game that simply stops.
At Little League and travel ball. Little League plays six innings, travel ball usually six or seven depending on age, and both add time limits on top because tournaments have to fit a schedule. That single change alters the whole shape of the game: a four run deficit in the fifth of a six inning game is much closer to over than the same deficit in the seventh of nine, and a coach managing a bullpen has two thirds of the room.
Batter's Eye: look up anything in baseball