Definition of "heyday" in English

noun

1. A period of success, popularity, or power; prime.

   • The early twentieth century was the heyday of the steam locomotive.

2. (archaic) An exultation of the spirits; gaiety; frolic.

interjection

1. A lively greeting.

2. (obsolete) An expression of frolic and exultation, and sometimes of wonder.

   • "Come follow me, my wags, and say, as I say. There's no riches but in rags; hey day, hey day, &c."