We know: the phrase "shared universe" sounds a whole lot like something that you'd learn about in first grade along with "the golden rule," "the buddy system," and "bring enough for the whole class."

But it's actually the most innovative part of a movie that puts good clean fun first and deep musings about life and the cosmos elsewhere.

But what in the name of Thor's hammer is a shared universe, and how did the term get started? The answers lie in the comic book company that Marvel refers to as the Distinguished Competition. (Source)

Comic Book Origins

DC Comics—or any of the smaller comic book companies they gobbled up on the road to total domination—were the first ones in on the comic book game, starting with Superman and continuing with Batman, Wonder Woman, Green Lantern and The Flash.

Because comics were cheap to draw and because comic book publishers had multiple superheroes in their stable, it wasn't hard to have one hero "guest star" in another one's magazine. It could help boost sales—putting Superman or Batman in another guy's comics would bring readers over—and, bonus, they're fun.

The first big one was the Justice Society of America in 1940, an All-Star Comics joint that featured nine of their biggest heroes (including later DC staples like Hawkman and The Spectre, as well as early versions of Green Lantern and the Flash). Superman and Batman began teaming up in their own comics fairly early as well, but the big game changer on that from was DC's publication of The Justice League of America in 1960.

It brought together the seven biggest characters in the DC universe—Superman, Batman, Wonder Woman, Green Lantern, The Flash, The Martian Manhunter and Aquaman—and it turned into a smash. (Source)

In fact, the Avengers themselves started out as a knock-off of the Justice League. Marvel had just been up and running for a couple of years, and they wanted something to boost sales like their competitors had.

Avengers #1 debuted in 1963, and featured Iron Man, Thor, the Hulk, Ant-Man and the Wasp. Captain America came along shortly thereafter and cemented the team, but by then, it was all just part of the comic book landscape

What About the Movies?

Writing team-ups and having shared universes in comic books is easy. The artist could just draw the heroes together and, as long as you had the rights to said heroes, it wouldn't cost you a dime.

But movies were much more expensive, and the idea of a team-up or crossover didn't hold a lot of financial appeal. (This is ironic, because television loved doing it, and even did it with superhero shows, as when Van Williams' Green Hornet teamed up with Adam West on the old Batman series.) (Source)

There was one big exception to that rule—though it wasn't a superhero movie.

Universal Pictures had enjoyed a huge success with their monster movies in the 1930s, and as they hunted for bigger and bigger gimmicks to lure in audiences, their standalone monster movies merged. Their stars had appeared in horror movies before—Boris "Frankenstein's Monster" Karloff and Bela "Dracula" Lugosi both starred in The Raven and The Invisible Ray, for example—but the formal monsters didn't meet up until Frankenstein Meets the Wolf Man in 1943. That was followed up with The House of Frankenstein in 1943, which brought all of the monsters in the stable out to play.

And yet there was still some weirdness going on there that spoke to managed expectations (and budget costs). Karloff was in House of Frankenstein, but he didn't play the monsters, Lugosi ended up playing the monster in Frankenstein Meets the Wolf Man, and both films smack of gimmicks rather than actual storytelling. That may be one of the reasons why nothing else so ambitious was ever attempted in Hollywood until The Avengers.

Enter the MCU

Superhero teams did exist in the movies before—notably the X-Men, Marvel's other successful movie franchise—but they started out as a team. (Hugh Jackman's solo Wolverine movies and Ryan Reynold's Deadpool came along afterwards). But the notion of pulling four distinct franchises—Iron Man, Captain America, Thor and The Incredible Hulk—was loony tunes.

And Marvel had to set it up very, very carefully.

It started with the huge success of 2008's Iron Man, which showed that second-tier Marvel characters could make great movies too. (Before that, conventional wisdom in Hollywood held that Spider-Man, the X-Men and the Fantastic Four were the only Marvel characters worth making movies about.) With that in place, Marvel poo-bah Kevin Feige started laying the threads for an Avengers movie.

He did so very carefully, releasing Iron Man 2 before any other superhero movies, then a solo Thor film and a solo Captain America film one right after the other. If any of them had tanked, Marvel could reassess and make sure their big ambitions didn't turn into a giant pit sucking up all their cash. Thankfully it didn't, and The Avengers not only got to bring all of those separate franchises together in a big-budget way that did more than just coast on the gimmick.

And with that, a shared universe became a reality.

So Why Is That a Big Deal?

A shared universe is a leap of imagination that, as we've said, hasn't happened a whole lot in the movies. It's also incredibly difficult to pull off, since each hero has his or her own energy, and getting all of them to work in a coherent story can wind up being a serious problem.

As it turns out, the movie juggled all of its characters incredibly well (more on that in the "Screenwriting" section), which showed the rest of Hollywood the possibilities that it could achieve.

What kind of possibilities? Well, how about allowing one superhero to gracefully retire (as Robert Downey may be doing), while introducing a new one to take his or her place (like the introduction of the Scarlet Witch and the Vision in the Avengers sequel, or the Black Panther in Captain America: Civil War)? What about setting up blockbuster movies like TV shows, with a new entry every six months instead of a week?

Yeah. Those are some pretty form-busting possibilities.

The shared universe notion is the biggest thing about The Avengers, and one of the reasons why this idea will someday define Hollywood in the 2010s. At the end of the day, though, the reason it works so well may be very simple. There was always something so cool about Batman showing up in Superman's mag, or the Justice League teaming up to fight some threat bigger than any one person could fight.

A kid's imagination could make those crude comic book drawings look like something larger than life, but the movies tend to look threadbare when you have to take on that much visual information without some serious money backing you up.

This film had the money to make those four-color images come to life in a real way, and with Joss Whedon, it had a director who wouldn't lose track of the cool characters in the middle of it all. It's a pretty incredible combination, and thanks to it, "shared universe" is no longer just a fan fiction term.