adverb
Your hair is as long as mine once was. He was once a famous film star. That once revered leader has fallen
He has visited his family only once in all these years
We must settle the itinerary once and for all before we can make the bookings
We go to the theatre once in a while
conjunction
Once the bus comes, you'd best get on it straight away
noun
at once
Watson, come here at once. I'll be there at once
You cannot be in two places at once
US