The Collaborative International Dictionary of English v.0.48:

Peneplain \Pe"ne*plain`\, n. [L. paene, pene, almost + E.
   plain.] (Phys. Geog.)
   A land surface reduced by erosion to the general condition of
   a plain, but not wholly devoid of hills; a base-level plain.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

peneplain
    n 1: a more or less level land surface representing an advanced
         stage of erosion undisturbed by crustal movements [syn:
         peneplain, peneplane]