The Collaborative International Dictionary of English v.0.48:

Declivity \De*cliv"i*ty\, n.; pl. Declivities. [L. declivitas,
   fr. declivis sloping, downhill; de + clivus a slope, a hill;
   akin to clinare to incline: cf. F. d['e]clivit['e]. See
   Decline.]
   1. Deviation from a horizontal line; gradual descent of
      surface; inclination downward; slope; -- opposed to
      acclivity, or ascent; the same slope, considered as
      descending, being a declivity, which, considered as
      ascending, is an acclivity.
      [1913 Webster]

   2. A descending surface; a sloping place.
      [1913 Webster]

            Commodious declivities and channels for the passage
            of the waters.                        --Derham.
      [1913 Webster]

WordNet (r) 3.0 (2006):

declivity
    n 1: a downward slope or bend [syn: descent, declivity,
         fall, decline, declination, declension,
         downslope] [ant: acclivity, ascent, climb, raise,
         rise, upgrade]