noun, plural: molecules
A tiny or a very small piece (of something).
(biology or biochemistry) An extremely small particle or substance, especially at cellular or structural level, as a biomolecule.
In physics and chemistry, a molecule is an electrically neutral, group of atoms that can exist alone in free state while its characteristic properties are retained. The atoms comprising the molecule may be of the same kind (as in oxygen molecule made up of two oxygen atoms) or of different kinds (such as water molecule made up of oxygen and hydrogen).
In biology, particularly in biochemistry, a molecule is used less strictly that it can also refer to any minute particle such as charged organic molecules or to substances (called biomolecules) produced and occur naturally in living organisms such as proteins, carbohydrates, DNA, etc.