From The Collaborative International Dictionary of English v.0.48:

Molecule \Mol"e*cule\, n. [Dim. fr. L. moles a mass: cf. F.
   mol['e]cule. See 3d Mole.]
   1. One of the very small invisible particles of which all
      ordinary matter is supposed to consist.
      [1913 Webster]

   2. (Physics) The smallest part of any substance which
      possesses the characteristic properties and qualities of
      that substance, and which can exist alone in a free state.
      [1913 Webster]

   3. (Chem.) A group of atoms so united and combined by
      chemical affinity that they form a complete, integrated
      whole, being the smallest portion of any particular
      compound that can exist in a free state; as, a molecule of
      water consists of two atoms of hydrogen and one of oxygen.
      Cf. Atom.
      [1913 Webster]
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