A chemical reaction involving the combination of oxygen with any other substance. One of the most common, yet least desired, results of the process of oxidation is the rusting of iron, which consists essentially of the combination of oxygen with iron (typically in the presence of moisture). Oxidation is caused by the fact that, due to the chemical structure of oxygen, it will bond with nearly anything (basically, the oxygen atom is two electrons short of having a full outer "shell," and it will hunt mercilessly for them). Although this process is the basis for respiration, it also has deleterious effects on many substances, and is the cause of such unpleasantnesses as the souring of milk, the tarnishing of silver, the spoiling of wine, and the spoilage of food, not to mention rusting. In fact, it is the reaction of oxygen with human cells that is the basis of the aging process. (Interestingly, at one point early in the history of life on Earth, there was very little atmospheric oxygen. Nearly all of the vegetation inhabiting the planet breathed nitrogen or sulfur. When green plants evolved and began emitting oxygen as a waste product, it built up to heavy concentrations in the atmosphere and killed off many of the earlier anaerobic life forms, to which oxygen was a potent poison. Oxygen, then, was the original air pollutant.) The word "oxygen" itself was named after the Greek word oxys, meaning "acid" and gen, meaning "to be born," as it was believed that oxygen was present in all acids (we now know this to be false).
In printing, oxidation is the process by which many inks dry. As the oxygen from the air is absorbed by and reacts with the substances in the ink vehicle, the fluid vehicle is hardened into a solid ink film. (See Drying Oil Vehicle.) Another chemical process called polymerization also contributes to hardening of ink films. Drying by oxidation should not be confused with drying by evaporation, which occurs when a solvent with a low boiling point simply turns into a gas and diffuses out of the ink, leaving the pigment behind. (See also Vehicle.)