The Collaborative International Dictionary of English v.0.48:

Downhill \Down"hill`\, adv.
   Towards the bottom of a hill; as, water runs downhill.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Downhill \Down"hill`\, a.
   Declivous; descending; sloping. "A downhill greensward."
   --Congrewe.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Downhill \Down"hill`\, n.
   Declivity; descent; slope.
   [1913 Webster]

         On th' icy downhills of this slippery life. --Du Bartas
                                                  (Trans. ).
   [1913 Webster]

WordNet (r) 3.0 (2006):

downhill
    adv 1: toward a lower or inferior state; "your performance has
           been going downhill for a long time now"
    2: toward the bottom of a hill; "running downhill, he gained a
       lot of speed"
    adj 1: sloping down rather steeply [syn: declivitous,
           downhill, downward-sloping]
    n 1: the downward slope of a hill
    2: a ski race down a trail

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "downhill":
   adown, aslant, aslantwise, aslope, at a slant, atilt, below,
   collapsing, deciduous, decline, declined, declining, declivate,
   declivitous, declivity, declivous, decurrent, descendant,
   descending, descent, dip, dipping, down, down south, down-reaching,
   downcoming, downfalling, downgate, downgoing, downgrade, downline,
   downright, downsinking, downstairs, downstream, downstreet,
   downtown, downward, downwards, downwith, drooping, drop, dropping,
   fall, falling, falling-off, hang, hanging, off plumb,
   on the descendant, on the downgrade, plummeting, plunging,
   rakingly, sagging, setting, sinking, slantingly, slantways,
   slantwise, slaunchways, slopeways, slopingly, submerging,
   subsiding, tipsily, tottering, tumbledown, upgrade, uphill