molecule

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The smallest particle of a substance that retains the chemical and physical properties of the substance and is composed of two or more atoms; a group of like or different atoms held together by chemical forces.
2. n. A small particle; a tiny bit.

Century Dictionary and Cyclopedia

1. n. The smallest mass of any substance which is capable of existing in a separate form — that is, the smallest part into which the substance can be divided without destroying its chemical character (identity). All the physical changes of a body, as the dissolving of sugar in water, the melting of lead, the change of water into steam, the magnetization of steel, and so on, are phenomena which take place without the loss of identity of the substance itself, and which concern the relations of the molecules among themselves. Hence the molecule is taken as the physical unit. A homogeneous body is regarded as made up of similar molecules, whose relations determine its physical qualities, and particularly its physical state as a gas, liquid, or solid. A gas, according to the kinetic theory of gases, is composed of molecules darting about in paths which are very nearly rectilinear through the greater part of their lengths. Liquids are supposed to be composed of molecules which wander about, but have not nearly rectilinear paths; while solids are believed to be composed of molecules bound together by cohesion and moving in quasi-orbital paths. A molecule of any substance is conceived as made up of one or more atoms, whose relations to each other are considered in chemistry. (See atom.) The exact nature of the molecules is still largely a matter of hypothesis, but as regards their size Sir William Thomson has reached a quasi-definite conclusion as follows: “If a drop of water were magnified to the size of the earth, the molecules or granules would each occupy spaces greater than those filled by small shot and smaller than those occupied by cricket-balls.”
2. n. A very small particle or bit of something; a particle; an atom.
3. n. In ornithology, the tread or cicatricula of a fecundated ovum.

Wiktionary

1. n. chemistry The smallest particle of a specific element or compound that retains the chemical properties of that element or compound; two or more atoms held together by chemical bonds.
2. n. A tiny amount.

GNU Webster's 1913

1. n. One of the very small invisible particles of which all ordinary matter is supposed to consist.
2. n. (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. n. (Chem.) 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. Cf. Atom.

WordNet 3.0

1. n. (nontechnical usage) a tiny piece of anything
2. n. (physics and chemistry) the simplest structural unit of an element or compound