One of the very small invisible particles of which all matter is supposed to consist.
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.
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.