downhill (Adjective) — Sloping down rather steeply.
downhill (Noun) — The downward slope of a hill.
downhill (Noun) — A ski race down a trail.
downhill (Adverb) — Toward a lower or inferior state. ex. "your performance has been going downhill for a long time now"
downhill (Adverb) — Toward the bottom of a hill. ex. "running downhill, he gained a lot of speed"