Definitions of downhill:

• noun:   a ski race down a trail
• noun:   the downward slope of a hill
• adjective:   sloping down rather steeply
• adverb:   toward the bottom of a hill

  Example: "Running downhill, he gained a lot of speed"

• adverb:   toward a lower or inferior state

  Example: "Your performance has been going downhill for a long time now"