What is MU not in physics?

The permeability constant (µ0), also known as the magnetic constant or the permeability of free space, is a measure of the amount of resistance encountered when forming a magnetic field in a classical vacuum. The magnetic constant has the exact (defined) value (µ0 = 4? × 10?7 H. read more

The physical constant μ 0, (pronounced "mu naught" or "mu zero"), commonly called the vacuum permeability, permeability of free space, permeability of vacuum, or magnetic constant, is an ideal, (baseline) physical constant, which is the value of magnetic permeability in a classical vacuum. read more

In general, permeability is not a constant, as it can vary with the position in the medium, the frequency of the field applied, humidity, temperature, and other parameters. In a nonlinear medium, the permeability can depend on the strength of the magnetic field. read more

#\mu# can describe a lot of quantities. Sometimes it's used in kinematics for coefficients of friction, or even in particle physics for the reduced mass of a particle. read more