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The preferred definition of glitch on Glitch City Laboratories when used as noun may refer to something happening or appearing in a program when it was not the developer's intention for it to work that way, be accessed that way, or be in the program itself.
The effects of glitches vary, from simple graphical glitches, game freezes, or sequence breaking, to strange Pokémon, attacks, or items that were never intended to appear in the game. Glitches may be harmful or harmless to a player's save file.
As a verbTo glitch, the verb, is a common slang term in the Pokémon glitch community that simply means to change something in a game to what it should not normally be. This usually happens after a glitch is done, creating the term.
As an adjectiveWhen used as an adjective, "glitch" indicates some in-game object that can only be acquired/triggered using a glitch. Examples include glitch Pokémon, glitch moves, glitch items, glitch encounter systems, etc. Sometimes a glitch object (especially a glitch Pokémon) would be simply referred to as a "glitch"; however, as of now, this usage of the noun "glitch" is discouraged.
The form glitched or glitchy may also be an adjective describing the state of a game after a glitching, or of a specific Pokémon or item affected by said glitching. A common example is a "glitched Hall of Fame" in Red/Blue versions, created after an encounter with Missingno., or a "glitched Kangaskhan" created through evolving the glitch Pokémon 'M (00).
Natural glitches, non-natural glitches, and glitch techniquesSome glitches are considered "natural glitches" because they can be performed from an unglitched game state, without having to make use of another glitch. If a glitch consists of multiple steps, the entire procedure is usually only considered a natural glitch if none of the intermediate steps apparently breaks the game. Natural glitches are always notable, because they indicate the existence of programming logic errors in the game.
The opposite of natural glitches are glitches that only happen under an already glitched game state. Strictly speaking, the behaviors under a glitched game state are all undefined, with none more "unintended" than the others. However, some of them are still notable, because they reflect unintuitive ways of the game dealing with unexpected data. For example, the Double Blue glitch, which is possible only with a sequence break, provides some interesting insight into the two scripted rival fights on Route 22.
Regardless of logical intuitiveness, some procedures are notable because they are useful, i.e. give the player an in-game advantage. Such a procedure is known as a "glitch technique", or a "trick". An example is the Celadon looping map trick, which allows the player to get almost any normal or glitch item from an expanded item pack. However, glitch techniques and glitches are not mutually exclusive. For example, the dry underflow trick is useful to setup the expanded item pack, but also shows an interesting quirk in the code that handles combining two item stacks.
ControversyThere is a degree of controversy of what is a glitch, such as whether intended mechanics that lead to unintended exploits are classed as glitches (such as date change exploit, possibly Bike Shop instant text glitch, go past the Marowak ghost without a Silph Scope), or whether accidental exploits (such as red bar manipulation) are glitches.
In the Pokémon speedrunning community, 'go past the Marowak ghost without a Silph Scope' is allowed in standard glitchless speedruns, despite it not being the developer's intention for the player to skip Marowak without the Silph Scope (and allowing this glitch became the convention). A later category known as "Any% Glitchless (Classic)" based on Japanese community rules bans using a Poké Doll on the ghost Marowak, as well as hard resets (which allow for luck manipulation).
Red bar manipulation is allowed but realistically is impossible to avoid without intentionally trying to avoid getting a Pokémon to low health. Bike Shop instant text glitch was once allowed as well, but later banned, however there are plans to allow it once again.
Some differentiate between "bug" and "glitch", for example possibly by identifying a bug as something that was clearly an error in the programming code, i.e. Game Boy ASM etc. (programming logic error with an example being the Focus Energy glitch) however the criteria for distinction between bug and glitch can be vague. Another distinction is of glitch being an error in computer hardware rather than software, however different understandings of 'glitch' were later socially constructed in video gaming communities.
There may also be controversy whether certain sequence breaks in video games count as glitches, or the access of placeholder data like ??????????.