A target area left uncovered; the sources divide the body into four openings, upper and lower on each side.
The four openings are upper right, upper left, lower right and lower left, divided by the belt and the centre line. Every guard covers some and exposes others, and every attack aims at one. Learning to see which opening is available, and which one you are exposing as you act, is the heart of reading an exchange.
Your familiarity