"DEFINITION of 'Economic Rent' An excess payment made to or for a factor of production over and above the amount expected by its owner. Economic rent is the positive difference between the actual payment made for a factor of production (such as land, labor or capital) to its owner and the payment level expected by the owner, due to its exclusivity or scarcity."

http://www.investopedia.com/terms/e/economicrent.asp

The definition is not that good, and the piece goes on to refine all rents as due to imperfectly competitive markets, (which is a bit nauseating - "Elvis markets" happen regardless of competition - substition breaks down, etc...).

But it is a start.