(1) A type of chemical reaction in which two or more molecules combine with the separation of water, alcohol, or other simple substance. The distinction from an addition reaction is not sharp, because reactions can be either stopped at the addition stage or carried a step beyond it. If a polymer is formed by condensation, the process is called polycondensation, e.g., phenolformaldehyde resin.See Claisen Condensation; Aldol.(2) The change of state of a substance from the vapor to the liquid (or solid) form.