Closer
The reliever who finishes games with a lead, and a job invented by a statistic.
What it actually is. The closer pitches the ninth inning when his team is ahead by three runs or fewer, and gets a save if he finishes the game without giving up the lead. He is normally the best reliever on the staff and he normally pitches one inning at a time.
What somebody who knows the game gets out of it. The closer role is the clearest case in baseball of a statistic creating a job. The save was defined in 1969, clubs started paying for saves, and within twenty years the best reliever on nearly every team was reserved for the ninth inning regardless of whether the ninth was the important inning. Leverage index makes the problem obvious: a one run lead in the eighth with the bases loaded matters far more than a three run lead in the ninth with nobody on, and the best arm is usually sitting during the first and pitching in the second.
The numbers worth knowing. Forty saves is an excellent season and the record is 62. A three run lead in the ninth with nobody on is held roughly 97 percent of the time by anybody, which is why the save is such a poor measure of quality.
What people get wrong about it. A save can be earned by pitching three innings with any lead, and it can be earned by getting one out with a three run lead and the bottom of the order coming up. Those are not remotely the same achievement and the statistic records them identically. Meanwhile the reliever who put out the fire in the seventh gets a hold, which almost nobody quotes.
At Little League and travel ball. There is no closer at youth level and trying to have one is usually a mistake, because the pitch count rules mean the arm you hold back may simply not be usable tomorrow. The travel version of the same decision is spending your best available arm in the inning that decides the game rather than saving him for a lead you may not have.
Batter's Eye: look up anything in baseball