Definition of heyday noun from the Oxford Advanced Learner's Dictionary

 

heyday

 noun
noun
BrE BrE//ˈheɪdeɪ//
 
; NAmE NAmE//ˈheɪdeɪ//
 
[usually singular]
 
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the time when somebody/something had most power or success, or was most popular synonym prime In its heyday, the company ran trains every fifteen minutes. a fine example from the heyday of Italian cinema a picture of Brigitte Bardot in her heyday Word Origin late 16th cent. (denoting good spirits or passion): from archaic heyday!, an exclamation of joy, surprise, etc.
See the Oxford Advanced American Dictionary entry: heyday