First time EVER starting a thread or whatever this is called. Here it goes:
Crossovers, the awesome stuff fan fiction is MADE and MADE for, I love seeing two worlds collide. But I'm just wondering, what defines a crossover? There are some crossovers that have taken the name and the genre and a few other elements but not the characters, none of the plot, none of the setting. Just the title and a few similar concepts. Is that a crossover? When you could pretty much change the title to anything and have it have NOTHING to do with the other franchise or idea you're crossing over, what is the point of calling it a crossover? I think there should be a more definitive way to decide what constitutes a crossover, exactly HOW MUCH is borrowed from two different stories to make one before it starts just applying to one or the other.

760269 In the barest sense, I'd say yes, that counts as a crossover. However, when deciding to list it as a cross or not, I'd firmly say no. If it's listed as "Crossover" I expect to see more than just "Character X uses weapon 4 from universe +".

The real question is... "What is a man? A miserable little pile of secrets! But enough talk... Have at you!"

In order for something to be a crossover, it needs to be a healthy 50-50 in my book. It needs to fit in both universes in some way it needs to make sense.

For example, for a crossover involving Castlevania, it just isn't a case where Simon Belmont will come out, whip ready to attack King Sombra, it needs to have a purpose. Another way a crossover can work is by taking a situation that happens in another universe and putting the pony twist on it, but I find it much more enjoyable for characters on both ends interacting with each other instead of Twilight Sparkle pulling out a whip and cross to take on Death.

Short definition: When the plotlines of two different canons are combined into a single work or art, in this case fan fiction.

Sensu Stricto, it's where canon characters from one setting interact with canon characters from the other. That's the strictest sense - the series literally "cross".
Broadest sense, anything which incorporates elements from more than one fictional setting. I would consider Best Night Ever to be at least potentially a crossover - with Groundhog Day. This definition would also count crossings of alternate universes, in some ways.

It depends a lot on how much you are going to put into it, but as said by Quick Fix, the elements of the story should make sense within the context of that story. As someone else said on a different thread or blog post, crossovers in themselves come in several forms, the most obvious ones of which are the 'meeting' type and the 'adaptation' type.

Meeting-type crossovers are generally one of a few subtypes. They are usually either 'worlds collide' where two different universes get blended into one another (The Conversion Bureau is a non-crossover example), 'characters meet' in which a group of characters go from one universe to another for an adventure (several Pokemon and most human in equestria such as Lero-verse) or 'everything is a single universe, they just haven't met' type, where everything is happening in one universe and chance brings things together (stargate crossovers, Star Trek crossovers).

Adaptation-type crossovers don't have any canon characters from one universe, instead they take elements of one universe and mix it into another, which can run from straight up replacement of characters to building an entire history where events lead to something happening (Fallout Equestria and Sweetie's Mansion being the latter).

In short, most anything that involves two or more 'worlds' being put into one story is a crossover of some type, just how much work the author has done defines just what type of crossover it is. I take Conversion Bureau and I have a character naturally become a dragon with magic but without 'wetwear' or 'cyberwear', I've got a Fireborn cross, particularly if I have said dragon remember things around the period Atlantis existed. I take a pony and have them run around a haunted mansion with a camera to fight off ghosts while trying to find what caused the place to be cursed with them and I've got Fatal Frame/MLP, on the other hand I have a starship from any space sci-fi universe find Equestria and the characters go planetside, there's another crossover.

As you can see, just about anything can classify, but it's the author that needs to make things work within the fic to have the fic be a viable crossover and gods know there's plenty of ones that have left me going "what the buck was happening there?".

What y'all have said does make sense, it seems we agree on the finer points of crossovers in that it has a broad definition and it's up to the author to blend all the ideas together to form a good story. Yes, there are a lot of terrible crossovers. We've all had the idea before. "I want character X from story X to meet character Y from story Y. I shall build a story around this idea!" and they generally don't put enough effort into it and the story is bland and leaves the reader with many wtbs. I try to answer the questions WHY world X and world Y are crossing over somewhere in the story, using borrowing established elements from one or both of the stories to make the connection.

I'm writing a Metal Gear crossover myself right now and I've found I could call it anything else, even say it's not a crossover at all. I could've left out many things from the MGS universe that I decided to implment for the sake of it being a crossover. Removing these things or even making the claim that they aren't applicable to Metal Gear would only be too easy. I decided to make it a crossover knowing that my audience would likely draw similarities to another franchise already. Rather it being Mechwarrior or something else involving giant robots, I chose Metal Gear. I've been pleased with the results so far.

I guess what makes a Crossover truly interesting is the character interaction - simply transplanting a character from one world into the other does not make a story, but having the characters of both canons interact with each other, or placing the characters of one canon in the role of the characters from the other canon to see what they would do differently, that is where a crossover truly shines.

In my opinon a crossover is between two series, with a more developed series (ex. Star Wars, Star Trek, Lord of the Rings) over riding the other.