Dictionary.net

A legal doctrine stating that if a person receives money or other property through no effort of his own, at the expense of another, the recipient should return the property to the rightful owner, even if the property was not obtained illegally. Most courts will order that the property be returned if the party who has suffered the loss brings a lawsuit.

1. Benefit retention with no consideration of return where it can be reasonably expected. 2. Money obtained that is not a gift that the beneficiary needs to make restitution for.