- an elementary particle having no charge, mass slightly greater than that of a proton, and spin of ½: a constituent of the nuclei of all atoms except those of hydrogen. Symbol: n
- a neutral elementary particle with a rest mass of 1.674 92716 × 10 –27 kilogram and spin 1/2; classified as a baryon. In the nucleus of an atom it is stable, but when free it decays
