The full form of μm is micrometre.
A micrometre (μm), also commonly referred to as a micron (though this term is becoming outdated), is a unit of length in the metric system equal to one millionth of a metre (0.000001 m). The symbol "μ" is the Greek letter mu, representing "micro-," which signifies one millionth. While sometimes incorrectly represented as "um," the correct abbreviation uses the Greek letter mu.
