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.