Micro is a prefix

SI prefix

The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...

 in the metric system

Metric system

The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

 denoting a factor of 10^-6 (one millionth

Millionth

One millionth is equal to 0.000 001, or 1 x 10-6 in scientific notation. It is the reciprocal of a million, can be also written as 1/1 000 000. Units using this fraction can be indicated using the prefix "micro-" from Greek, meaning "small"...

). Confirmed in 1960, the prefix comes from the Greek

Ancient Greek

Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...

  (), meaning "small".

The symbol for the prefix is the Greek letter μ (mu

Mu (letter)

Carlos Alberto Vives Restrepo is a Grammy Award and three-time Latin Grammy Award winning-Colombian singer, composer and actor.-Biography:...

). It is the only SI prefix which uses a character not from the Latin alphabet

Latin alphabet

The Latin alphabet, also called the Roman alphabet, is the most recognized alphabet used in the world today. It evolved from a western variety of the Greek alphabet called the Cumaean alphabet, which was adopted and modified by the Etruscans who ruled early Rome...

.

Symbol encoding in character sets

For historical reasons, there are two different characters in Unicode

Unicode

Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

, which appear slightly different in some fonts. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1

ISO/IEC 8859-1

ISO/IEC 8859-1:1998, Information technology — 8-bit single-byte coded graphic character sets — Part 1: Latin alphabet No. 1, is part of the ISO/IEC 8859 series of ASCII-based standard character encodings, first edition published in 1987. It is informally referred to as Latin-1. It is generally...

 (since 1985), at U+00B5 (Unicode 1.0, 1991). The Greek letter (μ) is encoded in the Greek range at U+03BC.

In circumstances in which only the Latin alphabet

Latin alphabet

The Latin alphabet, also called the Roman alphabet, is the most recognized alphabet used in the world today. It evolved from a western variety of the Greek alphabet called the Cumaean alphabet, which was adopted and modified by the Etruscans who ruled early Rome...

 is available, the prefix can be represented using the letter u

U

U is the twenty-first letter and a vowel in the basic modern Latin alphabet.-History:The letter U ultimately comes from the Semitic letter Waw by way of the letter Y. See the letter Y for details....

 as in um for μm, or uF for μF. In some contexts, such as pharmacy

Pharmacy

Pharmacy is the health profession that links the health sciences with the chemical sciences and it is charged with ensuring the safe and effective use of pharmaceutical drugs...

, microgram

Microgram

In the metric system, a microgram is a unit of mass equal to one millionth of a gram , or 1/1000 of a milligram. It is one of the smallest units of mass commonly used...

is often written "mcg", which is nonstandard.