Leakage inductance is that property of an electrical transformer that causes a winding to appear to have some pure inductance in series with the mutually-coupled transformer windings.
Usually, this is an undesireable property, but it is sometimes deliberately introduced into a transformer that is used as a ballast for a gas discharge lamp such as a neon sign, fluorescent lamp, or a high-intensity discharge lamp. In this case, the leakage inductance limits the current flow to the desired magnitude. (Without the leakage inductance, the negative resistance characteristic of these gas discharge lamps would cause them to destroy themselves by conducting too much current.)
Leakage inductance is primarily controlled by the design of the magnetic core used to form the transformer.