Placeholder

A player encounters 'MISSINGNO.' through the use of a glitch known as the 'Old Man Trick'. The name 'MISSINGNO.' refers to data in between legitimate. Nintendo have decided to refer to these unused Pokémon (which are actually Glitch Pokémon) as 'Missing Numbers'

Generally, the term 'placeholder' refers to any elements of a game which have been used as fillers in between existing data within a particular byte.

Often, various placeholders can be considered as beta elements within that game because they can contain data which is normally inaccessible through ordinary gameplay. In this case MISSINGNO.'s name is perhaps one of the most known examples.

However, not all placeholders are inaccessible because sometimes they can be useful for a developer to refer to data which the player has not 'unlocked' or 'encountered' yet. For example in the Gameboy Advance versions of the Pokémon games, even though a picture of a white question mark within a black circle (which is surrounded by a larger white circle) is used to account for several thousand Glitch Pokémon named ?????????? (which Glitch City Laboratories refer to as 'Decamarks' in normal gameplay, the game also uses this sprite to account for up to 386 unseen Pokémon in the Pokédex in order to occupy those in the Pokédex which the player hasn't seen yet.