What is leakage voltage?

There is no such thing.

What there is is leakage current, which is the amount of undesired current flow in a branch of an actual circuit when that branch of the equivalent ideal circuit would have no current flow.

Leakage current can also be used to refer to current that occurs on a path where there is no intentional circuit branch (e.g. corona discharge current to the air in a high voltage system).