adjective In the state of being active, functioning or operating.
All the lights are on, so they must be home.
adjective Happening; taking place; being or due to be put into action.
We had to ration our food because there was a war on.
adjective Of a person, used to express agreement to or acceptance of a proposal or challenge made by that person; most commonly with subject "you" (see you're on).
"Five bucks says the Cavs win tonight." ―"You're on!"
adjective Fitted; covering or being worn.
Your feet will soon warm up once your socks are on.
adjective Of a stated part of something, oriented towards the viewer or other specified direction.
The photograph shows the UFO side on.
adjective Acceptable, appropriate.
It's not fair to do that – it's just not on.
adjective Possible; capable of being successfully carried out.
Climbing up that steep ridge isn't on. We'll have to find another route.