once

Concise Oxford English Dictionary © 2008 Oxford University Press:
once/wʌns/

▶adverb

• 1 on one occasion or for one time only.

  ■ [usu. with neg.] at all; on even one occasion.

• 2 formerly.

• 3 multiplied by one.

▶conjunction as soon as; when.

– phrases
all at once

• 1 suddenly.

• 2 all at the same time.

at once

• 1 immediately.

• 2 simultaneously.

for once (or this once) on this occasion only.
once again (or more) one more time.
once and for all (or once for all) now and for the last time; finally.
once (or every once) in a while occasionally.
once or twice a few times.
once upon a time at some time in the past.

– origin ME ones, genitive of one.