The Wikipedia definition of “information leakage” is what happens “whenever a system designed to be closed to an eavesdropper reveals some information to unauthorized parties nonetheless.” I believe this to be an adequate although somewhat narrow definition. It assumes that the leakage is the result of a benign event, as opposed to an act of theft.

In the end it really doesn’t matter. Whether by accidental loss or by theft, your information is at risk while the systems to protect it are often non-existent.