Point of Divergence

A point of divergence is a specific event in an alternate history story that occurs differently than it did in factual history. Most alternate history authors will change a single event, creating a "ripple effect" that sets up the scenario. This divergence, which acts as the story's "unicorn in the garden" or "one big lie", may range in importance and realism from a character dropping or not dropping a piece of paper at a crucial time, to time travel and alien invasion (such as Harry Turtledove's "Balance" series, when World War II is interrupted by a fleet of lizard-people from outer space).

Film

 * Captain America: The First Avenger shows what happens when Allies and the Axis powers alike try to create a super soldier. You can see the point of divergence is when Steve successfully completes the experiment The Allies succeed, and the Axis powers opt to find a mysterious blue cube that can improve weapons development instead until Dr. Zola While Howard Stark fails to use the Tesseract to generate any powers, SHIELD learns it can power fighter jets, and Tony gets inspired by his father to develop clean energy using a larger version of the Arc Reactor.

Literature

 * In Witch Week, a book in the Chrestomanci series, it turns out this parallel universe of witches, prosecution and rampant magic was created because
 * Scott Westerfeld's Leviathan shows how World War I would have been kicked off much sooner and with more consequences. How? Charles Darwin discovered strands of DNA along with the theory of evolution, and he had the means to learn to create beasts by manipulating the helixes. An arms race ensues because the British are making giant living warships, while the Austrian-Hungary Empire focuses on robotics. Alex, instead of being one of many siblings, is an only child and orphaned after his parents Archduke Ferdinand and Catherine are killed, and the Russian Revolution never happens.
 * Dean Koontz's Lightning asks, "What if the Nazis had discovered time travel, and there were high-level resistance members seeking to ensure that Nazis never win World War II?" You get a bunch of time-traveling Nazis that decide to go after a woman in the 1980s that one of the Resistance members keeps saving.

Live-Action TV

 * In The Boys (series), this is how Supes emerged. A Nazi scientist defected during World War II with his wife, and founded Vought to profit from pharmaceuticals and build superheroes. Frederick Vought received a pardon from the US government to develop Compound V, and turned Supes into an entire industry where his company would manufacture heroes, and ideally super soldiers. As a result, there are no Marvel movies and Vought is the major corporation in the United States for entertainment, heroes, and merchandise.
 * Abed references this trope in an episode of Community where Jeff rolls a die to determine who will get the pizza for the study group's party. He says six different timelines can ensue, though Jeff rolls his eyes before tossing the die in the air. While things are okay if Annie, Pierce or Britta get the pizza, with Troy and Britta becoming Happily Married in one timeline, things become sad if Abed goes, and downright Nightmare Fuel in the timeline where Troy gets it. Fortunately,
 * Doctor Who
 * In the "better" world in "Doomsday," where Rose's dad Pete is still alive and successful in his business ventures, he explains that the Cybermen invasion was their divergence from the prime universe where Rose is the protagonist with Ten. After the chaos, the British government took control of Torchwood and made it more hospitable, rescuing the Doctor from Prime Torchwood.
 * Parallel Pete himself is a divergence. Rose knows him as her dad who died young and poor, and the greatest dad she knew. Unlike Prime Pete, this Pete found success with his ventures and inventions, and he never had children with Jackie. While he claims he's not father material when Rose reveals who she is and tearfully calls him "Dad," he bonds with her nevertheless and is teary-eyed on realizing that her mother is alive, after losing his Jackie in his debut episode.
 * Like in the video game, we see that the point of divergence in The Last of Us (series) is when the Cordyceps virus turns people into mindless zombies. While southern Texas is prepared, the rest of society isn't, and as of 2023, the whole world is a wasteland. One Indonesian scientist advises the government to raze the mainland and leave no survivors, because she can see how fast the spores spread. No more elections, no WiFi, and definitely not television anymore.

Video Games

 * The Civilization franchise makes these points of divergence more complicated than simply changing one fixed point; the player can manipulate different parts of the game to achieve their desired result. You can play many nations and make them the most powerful in history. Gandhi may even get into nuclear weapons and violence!
 * The Last of Us shows that in 2003, the fungus Cordyceps evolved into a mutant strain that turns people into mindless zombies. As a result, society has completely broken down by the time the game starts, to the point that Ellie in the DLC doesn't know what an arcade is.
 * It's unknown as of 2023 what the timeline divergence is specifically between Undertale and Deltarune since the start of the latter game greatly resembles the Golden Ending of the former, but the monsters don't know each other. A point of divergence did happen, though, as you can find references to "DETERMINATION" in Deltarune's second chapter while nudging Alphys and Undyne together.
 * Where in Time Is Carmen Sandiego? is all about how Carmen sends her crooks to specific parts of history to steal important relics and cause these points, for the fun of it. As a time sleuth, it's your job to get history back on track by helping complete puzzles.

Western Animation

 * In The Batman, Clock King's origin starts from him being jailed for a disproportionately long time because a petty theft attempt went wrong, and the trauma made him reverse time by a few seconds. The Bat family pities him when they hear his story, but they try to stop him from gassing all of Gotham City.
 * Danny Phantom
 * In the Dark Danny two-parter, an older Valerie and Dark Danny reveal to Prime Danny, Tucker and Sam that their Bad Future in Amity Park happened because
 * In another episode, Vlad blackmails Danny to cure his ghost acne by infecting Tucker and Sam, forcing the Fentons to quarantine them. Danny attempts to convince Clockwork that if Vlad never suffered the accident that gave him ghost powers, then it would save Sam and Tucker and maybe Vlad would become a better person.
 * Time Squad is all about this, where the title characters find out how to get history back on track for significant historical events. At one point, Eli Whitney didn't invent the cotton gin but robots instead, and they have to convince him that sorting cotton would be much safer.