Webmi·cron also mi·kron (mī′krŏn′) n. pl. mi·crons or mi·cra (-krə) also mi·krons or mi·kra (-krə) A unit of length equal to one thousandth (10-3) of a millimeter or one millionth (10-6) of a meter. Also called micrometer. [From Greek mīkron, neuter of mīkros, small.] American Heritage® Dictionary of the English Language, Fifth Edition ...