Definition of proton in US English:



  • A stable subatomic particle occurring in all atomic nuclei, with a positive electric charge equal in magnitude to that of an electron, but of opposite sign.

    The mass of the proton is 1,836 times greater than that of the electron. The atoms of each chemical element have a characteristic number of protons in the nucleus; this is known as the atomic number. The common isotope of hydrogen has a nucleus consisting of a single proton

    • ‘The strong force also attracts protons to protons or neutrons to neutrons.’
    • ‘An alpha particle consists of two protons and two neutrons; it is the nucleus of a helium atom.’
    • ‘Its atoms have a nucleus of two protons and two neutrons, orbited by two electrons.’
    • ‘Overall, atoms are neutral and the number of protons in their nuclei equals the number of electrons.’
    • ‘The matter was composed primarily of an ionized gas of electrons, protons, and helium nuclei.’


1920s: from Greek, neuter of prōtos ‘first’.