The Collaborative International Dictionary of English v.0.48:

Toponym \Top"o*nym\, n.
   1. A name of a place.
      [Webster 1913 Suppl.]

   2. a name, as in the binomial name of a plant, based on, or
      derived from, a place name, or based on the location of
      the thing named.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

toponym
    n 1: the name by which a geographical place is known [syn:
         place name, toponym]