Chase Rate
Swings at pitches outside the strike zone.
What it actually is. How often a hitter swings at pitches outside the strike zone.
What a scout reads off it. The truest read on plate discipline at any level, and it is a skill that shows up early and holds. A hitter who does not chase forces pitchers into the zone, which is where his power lives; a hitter who chases is pitched to entirely off the plate and never sees anything to hit.
At Little League and travel ball. This is the thing worth coaching at youth level above almost anything else, and it can be measured with a notepad rather than a camera: count the swings at pitches out of the zone in one game.
Batter's Eye: look up anything in baseball