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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]