- an elementary particle having no charge and having a mass and spin equal to that of the neutron but with magnetic moment opposite to that of the neutron; the antiparticle of the neutron.
- opposing the building, stockpiling, or use of neutron bombs: The antineutron lobby was outnumbered.
- the antiparticle of a neutron; a particle having the same mass as the neutron but a magnetic moment of opposite sign
