(1):
(n.) 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.
(2):
(n.) One of the very small invisible particles of which all matter is supposed to consist.
(3):
(n.) 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.