Oxidation is a reaction of enzymes in tea leaves with oxygen. Earlier it was also called fermentation but this term is incorrect because a fermentation process always involves microorganisms. In production of pu-erh tea fermentation is used to make the tea ripe.
Oxidation is an important part in production of tea. For green tea, the oxidation must be stopped immediately while black tea is oxidized to nearly 100%. Oolong however, has an average degree of oxidation of 10-80%.