Cut content refers to content that is not used (cut) either directly or indirectly in the final version of the game. From debug menus to unused music, graphics, enemies, or levels, many games have content never meant to be seen by anybody but the developers — or even meant for everybody, but cut due to time/budget constraints, not being interesting or unknown reasons. Most of it can still be found in the game files but is inaccessible within the game itself.
Note: Cut content should not be confused with legacy content, as the two differ in certain areas (with the exception of certain elements being removed, such as removed textures or scripts. Legacy content consists of the content from an earlier game that has not been modified in any manner. Cut content can consist of legacy content that has been adjusted for the later game (example:Nuka-Cola Quantum in the base Fallout: New Vegas game).