Placeholder

Generally, the term 'placeholder' refers to any element of a game which has been used as a filler for data in between existing data within a particular byte.

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 Pokémon the game refers to these unused Pokémon (which are actually Glitch Pokémon) as 'Missing Numbers'

Often, various placeholders can be considered as beta elements within that game because they can contain data which is 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 that 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.