Samuel Taylor Coleridge, the poet and author, called drama "that willing suspension of disbelief for the moment, which constitutes poetic faith ..."
Any creative endeavor — certainly any written creative endeavor — is only successful to the extent that the audience offers this willing suspension as they read, listen, or watch. It's part of an unspoken contract: The writer provides the reader/viewer/player with a good story, and in return, they accept the reality of the story as presented, and accept that characters in the fictional universe act on their own accord.
In other words, an author's work does not have to be realistic. It only has to be believable and internally consistent (and even the last requirement can be relieved to some extent). When the author pushes an audience beyond what they're willing to accept, the work fails in the eyes of that particular audience. Viewers are usually willing to go along with creative explanations for things, which is why people don't criticize your faster-than-light wormhole travel system or wonder how a shrinking potion doesn't violate the laws of matter conservation.
But even in the more fantastical genres, suspension of disbelief can be broken when a work breaks its own established laws or asks the audience to put up with too many things that come off as contrived. A common way of putting this is "You can ask an audience to believe the impossible, but not the improbable." For example, people will accept that the Grand Mage can use a magic spell to teleport across the world, or that the spaceship has technology that makes it completely invisible without rendering its own sensors blind. But the audience won't accept that the ferocious carnivore just happened to have a heart attack and die right before it would have killed the main character, or that the hacker guessed his enemy's password on the first try just by typing random letters. Without some prior detail justifying it or one of the Rules listed below coming into play, an audience is going to cry foul, and their suspended disbelief is now at the forefront of their mind. What is impossible in Real Life just has to be made the norm in the setting and kept consistent for an audience to accept it.
Of course, different people will have different thresholds for what they're willing to accept in a work, and what may break one person's willing suspension of disbelief may not necessarily have the same effect on another.
Most action movies push this trope almost to the breaking point; for the sake of action, the heroes can do virtually anything, given enough Phlebotinum.
As always, the various Rules override nearly all other considerations. When the audience's disbelief, which was suspended during the show, gets reinstated some time afterward, what you get is Fridge Logic. When the means of "fixing" the Plot Hole just end up making it worse and/or opening up a new one, that's a phenomenon known as the Voodoo Shark.
The MST3K Mantra is an exhortation to reinstate your Willing Suspension of Disbelief even if it's been broken, because "it's just a show". Similarly, Bellisario's Maxim calls the audience to reinstate their WSOD by ignoring whatever Plot Holes or other inconsistencies broke it, or that those things really aren't as important as the audience member(s) may think.
- Acceptable Breaks from Reality: Sometimes absolute realism can make a work or story boring, disgusting, downright impossible, or otherwise just doesn't work. Breaking from it in those instances, as long as it's not an Ass Pull and is internally consistent, actually makes your story better.
- The Anthropic Principle: Pretty much every story has some fundamental elements which, unrealistic or improbable as they may be, are vital for the story to function in the first place. So like them or not, you don't really have a choice but to suck it up and accept them if you want to enjoy the story. Sure, it might be unlikely that the Great Detective would just happen to be around to solve the incredibly byzantine murder that someone's committed close by, but if he or she wasn't there you wouldn't be able to enjoy watching him or her solve the mystery in the first place.
- Artistic License: As above, sometimes the correct depiction of something wouldn't work for the story in some way (e.g. in a Cop Show, having everyone do paperwork and eat and drink coffee 90 percent of the time would likely bore the audience, which is expecting drama and action). Generally doesn't break Willing Suspension of Disbelief unless the following applies:
- The Artistic License is so inaccurate and jarring that it doesn't even make sense even in the embellished version of the field (e.g. a cop in said Cop Show doing an on the spot sniff test for DNA)
- The Artistic License is internally inconsistent (e.g. it's established cops in said Cop Show, despite it being over the top action, can't dual-wield rocket launchers as they are human beings of ordinary strength... and then, suddenly, everyone is dual-wielding rocket launchers and the cops are on the level of space marines.)
- Ass Pull: The biggest way you can break Willing Suspension Of Disbelief. Some subtropes of it will do it almost 100 percent of the time, specifically the Deus ex Machina and Diabolus ex Machina. Generally, this trope and its subtropes needs to be approached carefully and handled masterfully in any form of media where they appear, to avoid pulling the audience out of their suspension.
- Clap Your Hands If You Believe
- The Coconut Effect: Audiences are used to the more unrealistic version of this thing, so including the real version of it is just going to confuse them. Like it or not, you'd be better off not going the "historically accurate" route, and just sticking with what the audience knows.
- Conceit: There are some ideas in every story that just have to be accepted. For instance, if the story is about space explorers going to Mars to meet with aliens, the conceit is that there is life on Mars.
- Contrived Coincidence: If the coincidence couldn't believably happen in Real Life for some reason, try to figure out why and how it could, OR try to figure out an In-Universe explanation for it happening that makes consistent sense. Otherwise, this is right up next to the Ass Pull as a way to immediately break Willing Suspension Of Disbelief.
- Fourth Wall/No Fourth Wall
- Gambit Roulette
- Gameplay and Story Segregation
- How Unscientific!
- Kayfabe: A form of suspending one's disbelief exclusive to Professional Wrestling. The audience knows that the outcomes of these matches are predetermined, and that the wrestlers in the ring aren't really throwing each other around or smashing each other in the face. However, the audience accepts these facts, and enjoys the show regardless.
- Lampshade Hanging: If the characters also agree that something doesn't make sense, the audience will be reassured.
- Literary Agent Hypothesis
- Magic A Is Magic A: If you want to make the audience believe the impossible, you must make it consistent between iterations of said impossibility. For example, if it's established that a wizard can only summon beings weaker than itself, and then the newbie summons a full-blown Eldritch Abomination, you might need to Hand Wave it or establish before that they can pull that off.
- Necessary Weasel: When the audience knows that the trope is unlikely/impossible/unrealistic, but is willing to accept it because it's just become part of the genre. Sure, Faster-Than-Light Travel is impossible, but if it means that Space Opera can take us to some creatively interesting parts of the universe quicker than several thousand human lifespans, we're willing to suck it up and go along with it.
- Plausible Deniability
- Reality Is Unrealistic: People expect fiction to follow certain conventions and patterns; Real Life doesn't have any such restrictions.
- Sci-Fi Writers Have No Sense of Scale: In the world of the science fiction genre, things tend to be conveniently huge, or conveniently smaller than what they are in reality for the sake of keeping the suspension of disbelief going. A particularly common subtrope of this is the Asteroid Thicket, a dense collection of extremely close asteroids that are somehow not merging together, despite the fact that at this sort of proximity they'd be well on the way to forming a new planet in real life.
- The Treachery of Images
- Weirdness Magnet: One of the ways to explain strange coincidences is that protagonists just have to deal with weird stuff all the time.