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.
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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.
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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]