Exit Velocity
How hard the ball comes off the bat, in miles per hour.
What it actually is. Exit velocity: how fast the ball leaves the bat, in miles per hour, measured by the tracking system in every park since 2015.
What a scout reads off it. The cleanest single measure of how hard a hitter hits, and it stabilises faster than almost anything else, which makes it the first thing to check when a hitter's results and his contact disagree. Average exit velocity is fine; the top of a hitter's range, his hardest few balls, says more about the ceiling.
Where the lines are. 95 miles per hour is the line the game calls hard hit. The hardest balls ever tracked sit around 122.
Batter's Eye: look up anything in baseball