noun (n)

• a downward slope or bend(noun.object)

  Synonym:
  declension, declination, decline, descent, downslope, fall
  Antonym:
  acclivity, ascent, climb, raise, rise, upgrade
  

  source: wordnet30
• 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.(noun)

  source: webster1913