An Easter egg is a hidden message, image, sound, or feature that a programmer plants inside software, intended to be found by a curious user or by someone reading the code later. The Jargon File, the long-running lexicon of hacker slang, gives two senses: a message hidden in the object code of a program as a joke, meant to be found by people disassembling or browsing the code; and a message, graphic, or sound effect emitted by a program in response to some undocumented set of commands or keystrokes, intended as a joke or to display program credits. The name comes from the custom of the Easter egg hunt, where treats are concealed for children to find.
The most famous origin story is the Atari 2600 game Adventure, released in 1980. Atari did not credit its programmers, so designer Warren Robinett hid his own name in a secret room: a player who carried a nearly invisible one-pixel object to a particular wall could pass through it into a chamber displaying “Created by Warren Robinett.” Atari discovered the surprise only after the cartridge shipped, and rather than recall it the company embraced the idea, reportedly encouraging more such hidden signatures. The episode is widely cited as the moment the software Easter egg became a recognized tradition.
Easter eggs split roughly along the two Jargon File senses. One kind is for end users: an undocumented sequence that produces something playful. Word processors, operating systems, and games have shipped flight simulators, credits reels, and animations triggered by obscure menu clicks or key combinations. The other kind is for fellow programmers, tucked into source or object code, where only someone reading or disassembling the program will see it.
The web turned the Easter egg into a near-daily genre. Mozilla’s browsers have long answered the address about:mozilla with cryptic verses from the mock “Book of Mozilla.” Google became known for hidden tricks in its search results and tools, and many sites reward a typed code or a particular click with an animation. These web eggs are deliberately discoverable enough to be shared, which is part of their appeal.
The Jargon File notes that Easter eggs sit on a spectrum with related practices. A signature or credit hidden in code shades into a back door if the hidden behavior grants access rather than amusement, and audited or safety-critical software generally forbids them outright, since any undocumented behavior is a liability. The line between a charming hidden joke and an unwanted surprise is exactly the line that security review polices.
As culture, the Easter egg encodes a particular value of the programming community: that the people who build software are individuals with a sense of humor, and that code is a place where authorship and play can leave a mark. That ethos connects the practice to wider hacker culture and to other shared in-jokes, such as the Konami Code, that recur across decades of software.