Tunneling
Two pitches that look the same out of the hand and split late.
How it actually works. A hitter has to commit to a swing roughly 175 milliseconds before the ball arrives. Any two pitches that look identical up to that moment and separate afterwards are, for practical purposes, the same pitch until it is too late.
Why it is done, and when it is wrong. This is why arsenals are built in pairs rather than as collections of good pitches. A fastball and a slider that share a tunnel beat a better fastball and a better slider that do not, which is the most useful thing to understand about modern pitch design.
Batter's Eye: look up anything in baseball