A masquerade is a trope found in speculative fiction, in which a group of supernatural or otherwise extraordinary people hide their existence from the mainstream society in the given setting.[1] The term was first coined by Robert A. Heinlein's Methuselah's Children in 1958. Those hidden groups may form a wainscot society, wherein they live adjacent to mainstream society in a covert manner, but overall remain their own distinct entity with their own culture.[2]
In a fantasy context, it means that magic is hidden, whether in secret locations, such as Diagon Alley in Harry Potter, or by magical forces, such as the Mist in Percy Jackson, or a glamour placed on individuals.
That is typically done to avoid some type of mass panic that would result in the destruction of the magical world by far more numerous normal people fearing the unknown. Masquerade societies may seek to hide this information from outsiders, or they may be disbelieved due to ignorance, conspiracies, or consensus reality. In works of fantasy with horror aspects, such as H. P. Lovecraft's Cthulhu Mythos, the Buffyverse, or White Wolf Games' World of Darkness setting, the majority's ignorance of the fantastical elements of their world will be portrayed as being to their own benefit.
The trope is used not only in fantasy but also in science fiction and superhero stories. A common device to keep hidden events from the masses in science fiction is the existence of aliens, such as in the Men in Black series. The secret identities of superheroes are also a type of masquerade, and the only superheroes that show their true identity are ones who can keep their family safe or have nothing to protect.
The masquerade trope assists writers by adding additional tension to the story, as well as helping the story appear plausible. It also saves the author from needing to write an alternate history in order to explain the existence of magic.