WordNet (r) 3.0 (2006):

neutron
    n 1: an elementary particle with 0 charge and mass about equal
         to a proton; enters into the structure of the atomic
         nucleus

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "neutron":
   K-meson, Kern, NMR, alpha particle, antibaryon, antilepton,
   antimeson, atomic nucleus, atomic particle, baryon, beta particle,
   electron, elementary particle, graviton, lepton, meson, mesotron,
   nuclear force, nuclear magnetic resonance, nuclear particle,
   nuclear resonance, nucleon, nucleosynthesis, nucleus, photon, pion,
   proton, quark, strangeness, strong interaction, triton,
   valence electron



The Elements (07Nov00):

neutron
Weight: 1.6749286*10^-27kg
A neutral hadron that is stable in the atomic nucleus but decays into a
protron, an electron and an antineutrino with a mean life of 12 minutes
outside the nucleus. Neutrons exist in all atomic nuclei except normal
hydrogen. Reported in 1932 by James Chadwick.