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The "Konami code" is a cheat code originally designed to be used with an NES controller in Konami video games from the mid-1980s. Over the years it's found its way into many other game systems and OSes. It represents the buttons that must be pushed in this order:

Up Up Down Down Left Right Left Right B A

The code usually gives the player a full powerup or extra lives. It's also often called the "Contra code" because that game was considered so difficult that many believed it was impossible to win without using the cheat code.