[alt + 0 + 1 + 8 + 1] yields "µ"
The symbol µ (pronounced "mu") is a Greek letter that is often used as a symbol in scientific and mathematical contexts to represent a variety of different things.
One of the most common uses of µ is as a prefix to represent a unit of measurement that is equal to one millionth (1/1,000,000) of a base unit. For example, µm (micrometer) represents one millionth of a meter, while µs (microsecond) represents one millionth of a second. In some fields, such as physics and engineering, the symbol µ is also used to represent the coefficient of friction, which is a measure of the resistance to motion between two surfaces in contact.
µ is often a unit of measurement used in the "visible wavelength range" or more accurately referred to as, "the electromagnetic spectrum." [See <this post> for an explanation of the electromagnetic spectrum.