Definition of upon preposition from the Oxford Advanced American Dictionary

      

    upon

     preposition
    preposition
    NAmE//əˈpɑn//
     
    , NAmE//əˈpɔn//
     
     
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  1. 1 (formal) = on The decision was based upon two considerations. Although the word upon has the same meaning as on, it is usually used in more formal contexts or in phrases such asonce upon a timeandrow upon row of seats.
  2. 2…upon… used to emphasize that there is a large number or amount of something mile upon mile of dusty road thousands upon thousands of letters
  3. Idioms
    once upon a time
     
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    used, especially at the beginning of stories, to mean “a long time in the past” Once upon a time there was a beautiful princess.
    (almost) upon you
     
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    if something in the future is almost upon you, it is going to arrive or happen very soon The summer season was almost upon them again.
See the Oxford Advanced Learner's Dictionary entry: upon