Definition of upon preposition from the Oxford Advanced Learner's Dictionary

      

    upon

     preposition
    preposition
    BrE BrE//əˈpɒn//
     
    ; NAmE NAmE//əˈpɑːn//
     
    , NAmE//əˈpɔːn//
     
     
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  1. 1  (formal, especially British English) = 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 time androw 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. Word Origin Middle English: from up + on, suggested by Old Norse upp á.Idioms used, especially at the beginning of stories, to mean ‘a long time in the past’ Once upon a time there was a beautiful princess. 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 American Dictionary entry: upon