Source : Webster's Revised Unabridged Dictionary (1913)

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
      matter is supposed to consist.

   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.

   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}.

Source : WordNet®

molecule
     n 1: (physics and chemistry) the simplest structural unit of an
          element or compound
     2: (nontechnical usage) a tiny piece of anything [syn: {atom},
        {particle}, {corpuscle}, {mote}, {speck}]