An example of a test cell in Fallout 3, this one was used to test traps

Test cells are locations within the files of a game and are inaccessible during normal gameplay. They are used by developers to test the functionality of various aspects of gameplay, including vendors, chests, traps, textures, quests, etc. In the Fallout series of games, test cells are found in Fallout 3, Fallout: New Vegas and Fallout 4, as well as their respective add-ons.

For a list of test cells, see Fallout 3 test cells, Fallout: New Vegas test cells and Fallout 4 test cells.