Easter egg

In a technology context, an Easter egg is a hidden surprise baked into a product by a designer who doesn't necessarily expect that anyone will ever actually discover and appreciate it. Canonically the first Easter egg is considered Warren Robinett's crediting himself in 1980's [Adventure] for the [Atari 2600], running contrary to Atari company policy of having its game designers be anonymous and thus unpoachable. Robinett's Easter egg was difficult to trigger accidentally, requiring players of the game to pick up an invisible object and deliberately transport it across the game world to a difficult-to-reach location.

A rich tradition of fun hidden content has followed over the past 45 years or so!