one time and no more
They met only once
He shaves once a week
at some indefinite time in the past; formerly
There once lived a king
by one degree of relationship
second cousin once removed
even one time; ever
They've never once lost a match at home
once again/more
now again as before
They were back home once again
for one more time
once and for all/once for all
for the final or only time; conclusively
once in a while
occasionally
once or twice
a few times
once upon a time
used as a traditional formula to begin a children's story: at some time in the past
[Middle English anes, ones, genitive of an, on one1]