Cheat code
A hidden button sequence or command granting invulnerability, weapons or level select, usually built first as a testing tool.
Developers needed to reach stage seven without playing through stage six every time. Kazuhisa Hashimoto at Konami put ↑↑↓↓←→←→BA into the 1986 NES port of Gradius to get through his own game, then forgot to take it out. Doom (1993) had IDDQD for god mode and IDCLIP for noclip. Leaving codes in was equal parts sloppiness and marketing: magazines sold issues on printing them.
Once games grew developer consoles and online modes, cheating moved into commands and settings, and outside the game into trainers and cheat cartridges. These days entering a code typically disables achievements — the game lets you cheat, but it tells on you.