Konami Code

The code was first used in the 1986 release of Gradius for the NES and was popularized among North American players in the NES version of Contra, for which it was also dubbed both the "Contra Code" and "30 Lives Code".

The Konami Code was created by Kazuhisa Hashimoto, who was developing the home port of the 1985 arcade game Gradius, a scrolling shooter released on the NES in 1986. Finding the game too difficult to play through during testing, he created a cheat code to give the player a full set of power-ups (normally attained gradually throughout the game). The code was still present in the released Gradius after Hashimoto forgot to remove it. Players discovered and shared the code via word of mouth.

The Konami Code was thus included in the series' other sequels and spin-offs, with some key differences:

In the Super NES version of Gradius III, the original code destroys the player's ship. However, replacing ← and → with the L and R triggers of the SNES controller powers up the ship.
In the most recent iteration of the series—Gradius Rebirth on the Wii—the Wii Remote's 1 and 2 buttons fill in for A and B, respectively.
The code has been subsequently re-used in a large number of other games and a varied version of the code is now used in the Opera Browser to activate hidden advanced settings.

Wikipedia[en.wikipedia.org]
What s the Konami Code? (video)