While words like Collaboration may give a better and wider meaning to the term. In reality, they aren't really that much different from each other.
To give a better picture, the full name of this term would be "Fictional Crossover", which Wikipedia defined it as
"A fictional crossover (short: crossover) is the placement of two or more otherwise discrete fictional characters, settings, or universes into the context of a single story. They can arise from legal agreements between the relevant copyright holders, or because of unauthorized efforts by fans, or even amid common corporate ownership. Crossprogramming is a term used in broadcast programming."
Collaboration … With the full list of mix and match series used in pixiv.