The inadvertent distribution of data through uncontrolled holes (or leaks) in the security perimeter. The leak occurs because of a lack of proper security procedures, or because of lax enforcement of such procedures. Employees may not be aware of the potential damage that they cause when sending proprietary information outside of the organization.
Beaver, K. Don’t Spring a Leak. Information Security, [Online, Jan 2006], http://informationsecurity.techtarget.com/magPrintFriendly/0,293813,sid42_gci1154838,00. html.