n. A hard, round seed of any one of several leguminous plants, used by children for playing marbles; especially, a seed of Guilandina Crista, G. major, Ormosia monosperma, or Stizolobium urens, or one of the plants itself. They are usually distinguished from each other, as mato azul, mato amarillo, etc.n. A game of marbles played with these seeds.