Character-coding is a generalised practice in fiction, in which characters are given traits of a certain group of people, without explicitly naming the character as a member of this group. The common example of 'queer-coding' has been prominently used throughout history and especially in film, often used oppressively to give villains an 'effeminate' energy which is deemed to make them more 'sinister'.
Jafar in Aladdin, Hans Gruber in Die Hard, etc..
Autistic-coding is another example, common in comedy, whereby characters are given autistic social traits without ever naming the character as autistic or otherwise disabled.
This can be harmful because it is often used as a comic trope, leading society to view autistic traits as embarrassing and laughable, doing damage to the autistic community.
Big Bang Theory
Sheldon Cooper in "The Big Bang Theory" is heavily autistic-coded and almost always the butt of jokes.
(CW: own opinions on perceived autistic-coding in a British TV series, relating to characters depicted as mentally-unwell and later dangerous/violent, csa mention, Black Mirror 'Spoilers')
Several recent episodes of this pioneering series have turned me personally off the series. Firstly, Shut Up and Dance features an anxious young protagonist named Kenny who is initially depicted to seem naïve and easily manipulated, but later is revealed to have been watching child pornography.
Secondly, the recent interactive feature film "Black Mirror: Bandersnatch" features a storyline around two highly introverted video game designers who both have obsessive-compulsive habits and thoughts, both of them becoming completely dissociated from reality in the major plotlines of the film and the main character Stefan's key character twist leads to him murdering his own father and disposing of the body before releasing his video game.
These kinds of plot lines avoid the criticism of autistic representation by never labeling the characters as autistic, but make specific directorial and plot decisions to highlight autistic-like behaviours before then twisting the characters into villains. Specifically, in "Bandersnatch" there is a interactive cue in which the viewer is allowed to choose between two anxious tics during a stressful moment for the protagonist:
"bite fingernails" or "pull earlobe"
The resemblance of both of these to common autistic stims is perhaps lost on most viewers, but subconsciously it suggests that stimming is a sign of mental illness and, ultimately, dangerous/violent people.
- "What we end up seeing are things like Bandersnatch. It came so close to what I want to see in the genre but it ended up severely missing the mark. Bandersnatch is particularly distinct because it’s a choose-your-own-adventure (it also happens to be a Black Mirror “episode” and as someone who loves Black Mirror I am especially upset about this). It has you assume the role of the protagonist where you can make decisions for how the plot unfolds. It ends up being a horror about the character falling into a psychosis-like state where he realizes he has no choice about his actions and that there are multiple timelines. A lot of “horror of mental illness” ends up coming to the question of whether or not the character is “truly crazy” and that ends up being the source of horror. As someone who lives with mental illness: this is not the source of horror and depicting it as what is “horrifying” about mental illness is ableist and harmful."