Spin Rate
How fast the pitch rotates, in revolutions per minute.
What it actually is. How fast the ball rotates in revolutions per minute, measured off the hand.
What a scout reads off it. Spin only matters in relation to velocity and axis. High spin on a four-seamer fought against gravity longer and looks like it rises; low spin on a sinker lets it fall. The same reading is a strength on one pitch and a weakness on another, which is why a spin number on its own tells you nothing.
Where the lines are. A four-seam fastball around 2,400 revolutions per minute and up plays as a riding fastball; curveballs run higher still.
How it lies to you. Spin can be manufactured, which the 2021 crackdown on grip substances demonstrated by removing several hundred revolutions per minute from a number of pitchers overnight. Compare a pitcher's spin to his own history and check when it changed.
Batter's Eye: look up anything in baseball