No, not those...
...the ones authors put in books.
Formally, it's called an allusion, but "Easter egg" is modern term coined by the gaming industry. To make it simple, it is a secret clue or message (and I love those!), aside from the storyline, that an author hides in his or her book for fans to find. It could be subtle and only found by readers with the keenest knowledge or it could appeal to a broader audience. It could point to a future book or it could be a revelation about the author. There are no rules.
Yes, I have an Easter egg in my book, From Shards to Sea Glass. I put it there for those who know their Bible well, the Old Testament to be exact, to have an "Ah!" moment followed by a knowing grin.
Would you like to know what my "Easter egg" is?
Photo by Kelly Neil on Unsplash