n.
   - a minute portion of matter.
 - Physics  a subatomic constituent of the physical world, e.g. an electron, proton, neutrino, or photon.
 - Grammar  a minor function word that has comparatively little meaning and does not inflect, e.g. in, up, off, or over used with verbs to make phrasal verbs.
 
 
  Etymology
  ME: from L. particula ‘little part’, dimin. of pars, part-.