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1. on one occasion or for one time only:

〖通例文尾で〗1度, 1回(だけ) (!2回はtwo timesよりtwiceが好まれる; 3回以上は数詞＋timesで表す)

e.g. they deliver once a week.

(usually with negative or if) at all; on even one occasion (used for emphasis):

〖if/when節内で〗いったん, ひとたび

e.g. he never once complained

e.g. if she once got an idea in her head you'd never move it.

2. at some time in the past; formerly:

〖通例文中･文尾で; 状態動詞と共に〗(現在と対比して)かつて, 以前は; 〖形容詞の前で複合語として〗かつては…だった

e.g. He had once been an Army officer.

as soon as; when:

〖従位接続詞; しばしば完了形と共に〗いったん…すると; …するとすぐに (!onceの節中では未来のことでもwillを使わない)

e.g. once the grapes were pressed, the juice was put into barrels.

1. without warning; suddenly: all at once the noise stopped.

2. all at the same time: scared and excited all at once.

1. immediately: I fell asleep at once.

2. simultaneously: computers that can do many things at once.

on this occasion only, as an exception: He was glad that for once he had not listened.

proverb a person cannot change their fundamental nature: once a whiner, always a whiner.

one more time.

now and for the last time; finally.

denoting someone or something that is eternal, enduring, or constant. 1950s: from T. H. White's Once and Future King (1958).

see bite.

from time to time; occasionally.

a few times.

at some time in the past (used as a conventional opening of a story).

formerly: once upon a time she would have been jealous, but no longer.

Middle English ones, genitive of one. The spelling change in the 16th century was in order to retain the unvoiced sound of the final consonant.