The basic meaning of the prefix micro- is “small.” It comes from the Greek prefix mikro-, from mikros, meaning “small.” In English micro- has been chiefly used since the 19th century to form science words. It is the counterpart for the prefix macro-("large") in pairs such as microcosm/macrocosm and micronucleus/macronucleus. And micro- is also sometimes the counterpart for the prefix mega-, as in microvolt (“one millionth of a volt”) and megavolt (“one million volts”).
