The Collaborative International Dictionary of English v.0.48:

Acclivity \Ac*cliv"i*ty\, n.; pl. Acclivities. [L. acclivitas,
   fr. acclivis, acclivus, ascending; ad + clivus a hill, slope,
   fr. root kli to lean. See Lean.]
   A slope or inclination of the earth, as the side of a hill,
   considered as ascending, in opposition to declivity, or
   descending; an upward slope; ascent.
   [1913 Webster]

WordNet (r) 3.0 (2006):

acclivity
    n 1: an upward slope or grade (as in a road); "the car couldn't
         make it up the rise" [syn: ascent, acclivity, rise,
         raise, climb, upgrade] [ant: declension,
         declination, decline, declivity, descent,
         downslope, fall]