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]