Molecule, n.
Etymology: [Dim. fr. L. moles a mass: cf. F. molécule. 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.)
1 more senses
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.
Shades of meaning
Fernald has no entry of its own for molecule; he lists it under particle.
Where he writes about it:
A molecule is made up of atoms, and is regarded as separable into its constituent parts; as used by physicists, a molecule is the smallest conceivable part which retains all the characteristics of the substance; thus, a molecule of water is made up of two atoms of hydrogen and one atom of oxygen. — from particle