Entry 1 sense Webster, 1913 micron /mīk'-ränˌ/ · mic·ron · IPA /ˈmaɪkɹɒn/ 01 n. A measure of length; the thousandth part of one millimeter; the millionth part of a meter. 1. A measure of length; the thousandth part of one millimeter; the millionth part of a meter.(Physics)