used, especially at the beginning of stories, to mean 'a long time in the past' :
Once upon a time there was a beautiful princess.
used, especially at the beginning of stories, to mean 'a long time in the past' :
Once upon a time there was a beautiful princess.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005