Charm is a quantum number that is equal to the number of charm quarks in a particle minus the number of anticharm quarks. Charm quarks were the fourth flavor (type) of quark to be discovered, in 1974. The lightest particles with charm are the D mesons, which are approximately twice as heavy as the proton.

 

A charmed particle is any particle that contains the exotic charm (c) quark.

 

Charmonium is any meson consisting of a charm quark and an anticharm quark. The name was coined in 1974 by A. De Rujula in analogy with positronium. Various states of charmonium have been found, including the J/psi particle.