The Collaborative International Dictionary of English v.0.48:

Single tax \Sin"gle tax`\ (Pol. Econ.)
   A tax levied upon land alone, irrespective of improvements,
   -- advocated by certain economists as the sole source of
   public revenue.

         Whatever may be thought of Henry George's single-tax
         theory as a whole, there can be little question that a
         relatively higher assessment of ground rent, with
         corresponding relief for those who have made
         improvements, is a much-needed reform.   --A. T.
                                                  Hadley.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

single tax
    n 1: a system of taxation in which a tax is levied on a single
         commodity (usually land)