Strikeout
Three strikes, and the single largest change in how baseball has been played over the last eighty years.
What it actually is. Three strikes retires the batter. He is out immediately unless the catcher fails to catch the third strike and first base is open or there are two out, in which case he can run. In a scorebook a strikeout is written K if he swung and a backwards K if he took it looking.
What somebody who knows the game gets out of it. The strikeout rate has risen in nearly every season since the Second World War and nothing forced it: no rule change, no equipment change, no mound change caused the trend. It is the sum of pitchers throwing harder with better breaking balls, clubs carrying more relievers so nobody has to pace himself, and hitters accepting strikeouts as the price of swinging for damage. It is the clearest example in the sport of an equilibrium moving because both sides independently decided the trade was worth it.
The numbers worth knowing. The league now strikes out in roughly 22 percent of plate appearances against about 12 percent in 1960. A hitter above 30 percent has a real contact problem; under 15 percent is a genuine contact skill. A pitcher above 30 percent strikeouts is an ace level bat misser.
What people get wrong about it. A strikeout is not automatically an out. On an uncaught third strike the batter may run, and if the catcher does not throw him out he is on base with the pitcher still credited with a strikeout. This is the single most confusing rule in baseball to a new fan and it is the source of the phrase four strikeouts in an inning.
At Little League and travel ball. The uncaught third strike is the rule that decides youth games, because youth catchers block far fewer balls. Little League Majors uses it; most leagues below that do not, so the same play at 9U and 11U produces different results and parents will argue about it every weekend.
Batter's Eye: look up anything in baseball