Small; a factor of one millionth.
Greek mikros, small.
This form is common and highly productive. Often it has a straightforward sense of something small or low in quantity: microprocessor, an integrated circuit that contains all the functions of a central processing unit of a computer; microcosm (Greek kosmos, world), something that encapsulates in miniature the characteristic qualities of something larger; micronutrient, a chemical substance required in trace amounts for normal growth.
The meaning is often affected by association with microscope and its derivatives, relating particularly to things too small to be seen by the unaided eye: microfilm, film containing very small images of documents; microcyte, an unusually small red blood cell, associated with certain anaemias; micro-organism, a very small organism, especially a bacterium, virus, or fungus; microfossil, a fossil or fossil fragment that can only be seen with a microscope.
The sense is sometimes of some tool or technique that is designed to deal with things or quantities of small size, as with microscope itself, and also such terms as microsurgery, intricate surgery performed using miniaturized instruments and a microscope, and microtome (Greek tomos, slice, section), an instrument for cutting extremely thin sections of material for examination under a microscope.
Some terms refer to a localized area, to a limitation in scope, or to a focus on a restricted subject area (often the opposite of macro‑): microeconomics, the part of economics concerned with single factors and the effects of individual decisions; microbrewery, a brewery which produces limited quantities of beer, typically for consumption on its own premises; micromanage, to manage or control every part, however small, of some enterprise or activity.
A microphone (Greek phōnē, sound, voice) does not necessarily deal with small sounds; it was named by analogy with microscope, the implication being that it enlarges sound, a function that strictly belongs to other parts of the audio system.
Micro‑ is also used as one of the standard SI (Système International) decimal prefixes (see the entryNumber words), indicating a factor of one million, as in the units microgram, micrometre, and microsecond.