There is some confusion over whether something can be categorized as a secret, easter egg, or myth. All info about the differences can be read here.
A secret is a part of the game that is usually not intended to be accessed. For example, the buildings where cutscenes are animated often aren't normally accessible ingame, but they can be accessed through 3rd party trainers. The Andromada plane in Grand Theft Auto: San Andreas is a real vehicle used in a cutscene, but is never made available to the player, however, there is a secret way to obtain it.