Line Drive
The best batted ball in baseball, and the one the hitter has the least control over turning into a hit.
What it actually is. A line drive is a batted ball hit hard and on a flat trajectory, usually between about ten and twenty five degrees of launch angle. It is the batted ball type most likely to become a hit and the one every hitting coach is describing when they say hit it hard somewhere.
What somebody who knows the game gets out of it. Line drive rate is the most useful quick read on whether a hitter's results are real, because line drives fall in at roughly a .680 clip against .240 for ground balls. But it is also the least stable batted ball type season to season, which is a genuine puzzle: hitters do not repeat line drive rate nearly as well as they repeat ground ball or fly ball rate. The modern resolution is to stop classifying by shape and use exit velocity and launch angle directly, which is what expected batting average and barrels do.
The numbers worth knowing. Around 20 to 22 percent of balls in play are line drives. They produce a batting average near .680 and a slugging percentage over 1.000. A hitter with a line drive rate above 25 percent is squaring the ball up unusually often.
What people get wrong about it. A line drive caught is still just an out, and the scoreboard has no memory. This is the single largest source of the gap between how well a hitter is going and how well he is hitting, and it is why expected statistics exist: a week of hard line drives at fielders and a week of soft bloopers over them look identical in the box score and mean opposite things about next week.
At Little League and travel ball. Line drives are the correct target at every level including youth, and they are also the safest thing to teach because the swing that produces them is the swing that stays useful at every future age. A twelve year old hitting hard line drives is a better prospect than one hitting occasional home runs with a big uppercut.
Batter's Eye: look up anything in baseball