By Anne Marie Helmenstine, Ph.D. Micron Definition: A micron is a unit of length equivalent to a millionth of a meter. 1 micron = 1 μm = 10 -6 m. Also Known As: micrometer, micrometre, μm. Examples: Red blood cells are approximately 10 microns in diameter. Human hair is between 10 and 100 microns in diameter.