micrometer (mĪkrŏmˈətər, mĪˈkrōmēˌtər) [key]. 1 Instrument used for measuring extremely small distances. Typical examples are devices used in astronomical telescopes to measure the apparent diameter of celestial objects and similar devices used in microscopes. In both of these devices a fine hair or filament is moved from one extremity to the other of the image of an object and the distance read on a calibrated scale. Another typical micrometer is the micrometer caliper, a device in which an object to be measured is enclosed between two jaws, one fixed and the other movable by means of a fine screw. When the jaws are just touching the object, the distance between the jaws can be read on an associated scale, often to an accuracy of 10 - 4 (one ten-thousandth) in., or 10 - 6 (one millionth) m. 2 Unit of linear distance equal to 10 - 6 (one millionth) m. It was formerly known as a micron.