The Collaborative International Dictionary of English v.0.48:

Unearned \Un*earned"\, a.
   Not earned; not gained by labor or service.
   [1913 Webster]

   Unearned increment (Polit. Econ.), a increase in the value
      of land due to no labor or expenditure on the part of the
      owner, but to natural causes, such as the increase of
      population, the growth of a town in the vicinity, or the
      like. Some hold that this should belong to the nation.
      [1913 Webster]

WordNet (r) 3.0 (2006):

unearned increment
    n 1: an unearned rise in the market value of property resulting
         from general market factors