- a common replacement for µ- as a symbol for the SI prefix micro- (10-6).The symbol is frequently seen in combinations such as ug for the microgram (µg). The use of u- was approved by the International Organization for Standardization in its standard ISO 2955, issued in 1974. The problem at that time was that many of the character sets in common use did not include the Greek letter µ. This is much less of a problem today, so ISO 2955 has been withdrawn and is no longer in effect. The message from all standards agencies now is: use µ-, not u-, for micro-.
Dictionary of units of measurement. 2015.