Baryons are the family of subatomic particles with a baryon number of 1. The term baryon is usually use to refer to triquarks — baryons made of three quarks. "Exotic" baryons made of four quarks and one anti-quark, are known as the pentaquarks, but their existence is not generally accepted. Each baryon has a corresponding antiparticle (anti-baryon) where quarks are replaced by their corresponding antiquarks and their corresponding anti-quarks replaced by quarks. Amongst the baryons are the protons and neutrons, which make up atomic nuclei, but many other unstable baryons exist as well. The term "baryon" is derived from the Greek βαρύς (barys), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles.