Permeance, in general, is the degree to which a material admits a flow of matter or energy.
A magnetic circuit almost acts as though the flux is conducted, therefore permeance is larger for large cross-sections of a material and smaller for smaller cross section lengths.
using Hopkinson's law (magnetic circuit analogue of Ohm's law for electric circuits) and the definition of magnetomotive force (magnetic analogue of electromotive force):
where: Alternatively in terms of magnetic permeability (analogous to electric conductivity):
where: The SI unit of magnetic permeance is the henry (H), equivalently, webers per ampere.