Micro- (symbol µ) 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 comes from 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. Also the letter u instead of µ is allowed by one of the ISO documents.
Typical bacteria are 1 to 10 micrometres in diameter.
Eukaryotic cells are typically 10 to 100 micrometres in diameter.