μ is the Greek letter mu. It’s commonly used to mean micro- in science and engineering, which stands for one millionth (10−6)(10^{-6})(10−6), and in statistics it often means the population mean.

Common meanings

  • Micro- prefix: μmμmμm means micrometer, or one-millionth of a meter.
  • Statistics: μμμ usually represents the population mean.
  • Physics: it can also stand for coefficient of friction or magnetic permeability.

In plain English

If you see μ in a math or science context, the meaning depends on the subject. For example, in 5 μm5,μm5μm it means “micro,” but in a stats formula it usually means “mean”.

Tiny example

  • 3 μs3,μs3μs = 3 microseconds = 3 millionths of a second.

TL;DR: μ = mu , and it most often means micro- or population mean depending on context.