adverb
one time only
at any one time :ever
at some time in the past
once again/more
for another time :one more time :again
once and for all
now and for the last time
once bitten, twice shy
once in a blue moon
see blue moon
once in a while
sometimes but not often :occasionally
once upon a time
at some time in the past
Once upon a time is the traditional way to begin a fairy tale.
noun
[noncount] :one single time
at once
at the same time
right away :without delay :immediately
conjunction
at the moment when :as soon as