The lowercase Greek letter mu (µ) is used to represent the prefix multiplier 0.000001 (10 -6 or one millionth). For example, 0.000000001 farad or 10 -9 F of capacitance is commonly written as 0.001 µF.
The symbol µ is sometimes used to represent permeability, an expression of the extent to which a substance concentrates magnetic lines of flux. In some texts, the symbol µ is an abbreviation of micrometer(s) or micron(s). These two terms both refer to a unit of displacement equal to 0.000001 meter or 0.001 millimeter.
In most IBM-compatible computers, the µ symbol can be generated by activating the NUM LOCK feature, holding down the ALT key, and then, while keeping ALT held down, entering 230 on the keypad at the right-hand end of the keyboard. When ALT is released after having entered these digits, the µ symbol appears. This works in virtually all Windows-based word processors, and even in some DOS programs.
Also see Mathematical Symbols.