Corrupt actions whose sole outcome is the supplementing of the income of the provider without affecting the "real world" in any manner.
Though the perception of corruption itself is a negative outcome - it is so only when corruption does not constitute an acceptable and normative part of the playing field. When corruption becomes institutionalised - it also becomes predictable and is easily and seamlessly incorporated into decision making processes of all economic players and moral agents. They develop "by-passes" and "techniques" which allow them to restore an efficient market equilibrium. In a way, all-pervasive corruption is transparent and, thus, a form of taxation.