Baryon, BAR ee on, is a unit of matter smaller than an atom composed of three still smaller particles called quarks. A baryon is one of two types of composite particles known as hadrons. The other type is called a meson. The name baryon comes from the Greek word for heavy. The lightest baryon, the proton, has a mass that is 1,836 times that of an electron. In general, the heavier the baryon, the more unstable it is--that is, the more rapidly it will decay (break down) into smaller particles.