1.
If something or someone is moving downhill or is downhill, they are moving down a slope or are located towards the bottom of a hill.
He headed downhill towards the river...
? uphill
ADV: ADV after v, be ADV, ADV from n
•
Downhill is also an adjective.
...downhill ski runs...
ADJ: ADJ n
2.
If you say that something is going downhill, you mean that it is becoming worse or less successful.
Since I started to work longer hours things have gone steadily downhill...
ADV: ADV after v, be ADV
3.
If you say that a task or situation is downhill after a particular stage or time, you mean that it is easy to deal with after that stage or time.
Well, I guess it's all downhill from here.
? uphill
ADJ: v-link ADJ