An unused block of data in the Game Boy games Pokémon Red Version and Green Version in Japan. (later upgraded into a summarized game called Pokémon Blue Version, which had its code changed slightly and shipped to North America as Pokémon Red Version and Blue Version) The data is read by the game as a Pokémon named MissingNo., or "Missing Number." Apparently used as possible beta testing material or a scrapped idea. A hole in the game data was discovered allowing people to find it in the wild and capture it, causing major glitches and programming errors in the game's coding. It is supposedly Pokémon number 000 in the National Pocket Monsters Index, but is not officially classified.
This soon became the most popular name for any "glitch" Pokémon data found in later games in the series. When Pokémon Yellow Version, the sequel to Red and Blue versions, was released, MissingNo. was not found in the data, and was locked out of the previous games from trading and battling. In the games Pokémon Gold Version and Silver Version, blocks of unused data were found via hacking with cheat devices, but froze the game at certain points. Similar data was found in Crystal Version, the sequel to Gold and Silver Versions, but it all had no use.
When Pokémon arrived on the Game Boy Advance under the titles Pokémon Ruby Version and Sapphire Version, people began hacking the near-perfect coding to find unused or glitched data in the game. Indeed, one such block of data was found. It was Pokémon data labeled "?" and had its own sprites, stats, moveset, and National PokéDex number of 390. It functioned properly as an actual Pokémon And was tradeable. Obviously it was there on purpose, but most suspect it is merely for beta testing, as it had very basic attributes.
When remakes of Red and Green Versions, Pokémon FireRed Version and LeafGreen Version, were released, the very same "?" from Ruby and Sapphire Versions was found in the game data, registered under the same number and stats. It is fully usable once hacked out of the game data, and is tradeable with Ruby and Sapphire Versions. The purpose is still unknown.
The sequel to Ruby and Sapphire Versions, called Pokémon Emerald Version, upgraded from Ruby and Sapphire Versions much like Crystal Version did to Gold and Silver Versions. The "?" has been discovered to be tradeable to it and the same data is stored in the previous GBA games is still in Emerald Version. Nobody knows Game Freak's plan for this data. Perhaps it was merely a beta test and kept in later games for compatability purposes.
Contrary to popular belief, the original "glitch" data for MissingNo. in Pokémon Red and Blue Versions is NOT in the remakes Pokémon FireRed or LeafGreen Versions. The data has since been perfected.