The unit of measurement "um" is an abbreviation for micrometer or micron, which is a length of measurement equal to one millionth of a meter. It is commonly used to measure microscopic objects like cells and the thickness of plastic in the packaging industry. One micron is equal to 1/25,400 of an inch or one-thousandth of a millimeter. In the packaging industry, a micrometer is used to measure the thickness of plastic to determine how many micrometers thick a sample of plastic is.