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Definition of heyday noun from the Oxford Advanced American Dictionary

     

    heyday

     noun
    noun
    NAmE//ˈheɪdeɪ//
     
    [usually singular]
     
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  1. 1the time when someone or something had most power or success, or was most popular synonym prime
  2. 2 In its heyday, the company ran trains every fifteen minutes. a fine example from the heyday of Italian cinema a picture of Marilyn Monroe in her heyday
See the Oxford Advanced Learner's Dictionary entry: heyday