A bug leakage results when a bug is detected which should have been detected in earlier builds/versions of the application.
A defect which exists during testing yet unfound by the tester which is eventually found by the tester/end-user is also called bug leakage.
A bug release is when a particular version of s/w is released with a set of known bug(s)/defect(s). These bugs are usually low severity and/or low priority bugs. It is done when the company can afford the existence of bug in the released s/w rather than the time/cost for fixing it in that particular version. These bugs are usually mentioned in the Release Notes.