Black tea is tea leaves that have been quickly and heavily oxidized. This gives the leaf a dark appearance and significantly changes the aroma and flavor.

Oxidation is the natural chemical reaction carried out by enzymes within the leaf that begin once they are exposed to air (like an apple turning brown when cut open).

Though all teas (other than green tea) go through some degree of oxidation, black tea is the only one that is defined by its high level of oxidation.