Definition - What does Easter Egg mean?
An Easter egg is a hidden video game feature or surprise. Easter eggs are usually unlocked by using certain techniques to complete in-game tasks, entering specific button combinations or acquiring access to secret game or game file areas.
Techopedia explains Easter Egg
Game developers often add Easter eggs with game producer consent. Unauthorized Easter eggs added without game producer knowledge or consent can lead to game recall, or the game may be re-rated to reflect hidden content.
Unauthorized content aside, most Easter eggs are harmless and enhance game player interest. One of the most common Easter eggs allows gamers to play as hidden characters. Many of these Easter egg types were included in the U.S. National Basketball Association's (NBA) "Jam" series, and were enjoyed by avid gamers and developers alike.
The first Easter egg was supposedly found in Atari's "Adventure" video game, released in 1978.