What is Reluctance in a magnetic circuit ?

Reluctance is the resistance to the flow of magnetic flux in a magnetic circuit.  Its unit is Henry^-1.  It is often compared to resistance in an electric circuit. However, unlike a resistance where energy is dissipated as heat.  The reluctance does not cause a dissipation of energy.
Reluctance is denoted by a capital 'R'.  The R being cursive to distinguish it from resistance.

It is also denoted as the ratio between the magnetomotive force and the magnetic flux.

Reluctance = Magnetomotive force / Magnetic flux

This relation is also known as Hopkinson's law and is analogous to ohm's law.

Materials which can be magnetised easily such as iron and steel have low reluctance while materials which cannot be magnetised have high reluctance.  The opposite of reluctance is permeability.