1) Unit of length 
The micrometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: µm) or micrometer (American spelling), also commonly known as a micron, is an SI derived unit of length equaling 1×10−6 of a metre (SI standard prefix `micro-` = 10−6); that is, one millionth of a metre (or one thousandth of a mil.....
metric unit of measure for length equal to 0.00l mm, or about 0.000039 inch. Its symbol is m. The micrometre is commonly employed to measure the ... [2 related articles]
One-millionth of a metre, or ten to the minus sixth metre (abbreviated um), the unit used for measuring microorganisms. ... (09 Oct 1997) ... 
See Micron.
One-millionth of a metre