A micrometre or micron (American spelling: micrometer; symbol µm)'s one millionth of a metre, or equivalently one thousandth of a millimetre. It can be written in scientific notation as 1×10−6 m, meaning . A strand of human hair's about 100 µm wide. Red blood cells are 7 µm in diameter.) are still used (especially in astronomy and the semiconductor industry) to denote a micrometre.… (More on Micrometre)