Wiktionary
n. An SI#Abbreviation/MKS unit of measure, the length of one millionth of a metre. Symbol: µm, um, rm
Wikipedia
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 of a metre (SI standard prefix " micro-" = 10); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch). The symbol μm is sometimes rendered as um if the symbol μ cannot be used, or if the writer is not aware of the distinction.
The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria and is also commonly used in plastics manufacturing. Micrometres are the standard for grading wool by the diameter of the fibres; wool finer than 25 μm can be used for garments, while coarser grades are used for outerwear, rugs, and carpets. The width of a single human hair ranges from approximately 10 to 200 μm.
Usage examples of "micrometre".
She hurt, she bled, but her colossal inertia drove her into the grit with the force of a missile, so that pieces only micrometres in diameter pierced straight through her at whatever vector she struck them.