heyday(noun, interjection) interjection1. A lively greeting.2. (obsolete) An expression of frolic and exultation, and sometimes of wonder. noun1. the period of greatest prosperity or productivitySimilar word(s): bloom, blossom, efflorescence, flower, flush, peak, primeDefinition categories: time, periodSentences with heyday as a noun:- The early twentieth century was the heyday of the steam locomotive.