Micro (Greek letter μ (U+03BC) or the legacy symbol µ (U+00B5)) is a unit prefix in the metric system denoting a factor of 10⁻⁶ (one millionth). Confirmed in 1960, the prefix comes from the Greek μικρός (mikrós), meaning "small".

The symbol for the prefix is the Greek letter μ (mu). It is the only SI prefix which uses a character not from the Latin alphabet. "mc" is commonly used as a prefix when the character "μ" is not available; for example, "mcg" commonly denotes a microgram. This may be ambiguous in rare circumstances in that mcg could also be read as a micrigram, i.e. 10⁻¹⁴ g; however the prefix micri is not standard, nor widely known, and is considered obsolete. The letter u, instead of μ, was allowed by an ISO document, but that document has been withdrawn in 2001, however DIN 66030:2002 still allows this substitution.