Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. A descending surface; a sloping place.

            Commodious declivities and channels for the passage
            of the waters.                        --Derham.

Source : WordNet®

declivity
     n : a downward slope or bend [syn: {descent}, {fall}, {decline},
          {declination}, {declension}, {downslope}] [ant: {ascent}]