Any unauthorised vertical deviation of more than 300 feet from an ATC flight clearance.
EUROCONTROL EATMP Glossary, June 2001
Within RVSM airspace this limit is reduced to 200 feet.(EUROCONTROL - HEIDI)
Definitions applied by other organisations are similar but sometimes refer to a deviation of 300 feet or more.
The level bust issue only relates to aircraft in controlled airspace or a designated ATZ outside controlled airspace and under either radar or procedural ATC control.
(Note source: Skybrary, URL: http://www.skybrary.aero/index.php/Level_Bust)