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.