In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically represented by the Greek letter μ. The term was coined in September, 1885 by Oliver Heaviside. The reciprocal of magnetic permeability is magnetic reluctivity.
We can simplify it by saying, the more conductive a material is to a magnetic field, the higher its permeability.
In SI units, permeability is measured in the henries per meter (H·m−1), or newtons per ampere squared (N·A−2). 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 = 4π×10−7 ≈ 1.2566370614…×10−6 H·m−1 or N·A−2).
A closely related property of materials is magnetic susceptibility, which is a measure of the magnetization of a material in addition to the magnetization of the space occupied by the material.