- Neutral nuclear particle of essentially the same mass as the proton, and together with protons accounting for most of the mass of atomic nuclei
- An elementary particle that has no electric charge, and whose mass slightly exceeds that of a proton. It is in all atomic nuclei except protium, which is hydrogen-1. It is a baryon, and when part of a nucleus is very stable. When free, a neutron decays in a few minutes into a proton, an electron, and an antineutrino.