Real Dictionary

proton
countable noun: A proton is an atomic particle that has a positive electrical charge.

proton in American English
a nucleon carrying a positive charge equal to the negative charge of an electron and having a mass of c. 1.673 × 10-27kg (c. 938.2796 MeV/c2, c. 1,836 times the mass of an electron): the number of protons in an atom determines the atomic number of a chemi
noun: a positively charged elementary particle that is a fundamental constituent of all atomic nuclei. It is the lightest and most stable baryon, having a charge equal in magnitude to that of the electron, a spin of 1⁄2, and a mass of 1.673 × 10−27kg

proton in British English
noun: a stable, positively charged elementary particle, found in atomic nuclei in numbers equal to the atomic number of the element. It is a baryon with a charge of 1.602176462 × 10–19 coulomb, a rest mass of 1.672 62159 × 10–27 kilogram, and spin 1⁄2