An Etymological Dictionary of Astronomy and Astrophysics

A unit of length in the → metric system equal to one millionth of a → meter (10^-6 m); symbol μm. Also called → micrometer. The average thickness of a human hair is about 50 μm (30-100 μm). The human eye cannot see anything smaller than 40 μm in size. Other small sizes: white blood cells = 15 μm; red blood cells = 8 μm; bacteria 2 μm.

See also: Coined 1880 in Fr. from Gk. mikron, neutral of mikros “small.”