Masquerade

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The common TVTropes endorsed name for a phenomena in a lot of Urban Fantasy: The supernatural is kept secret to the public, thus allowing the story to be set in something resembling the "real" world.

Why and how the supernatural is kept secret varies from work to work. Just from /tg/ related stuff:

• In Vampire: The Masquerade (from which the TVTropes trope gets its name) and Vampire: The Requiem, you're fucking vampires, social parasites on humanity. You don't want your prey to start hunting you.
• In Mage: The Awakening, the Masquerade is supernaturally enforced; any mundane (called "Sleeper") who sees magic causes magic to go awry, damaging the caster.
• The Skaven had something like this going on in the original Warhammer Fantasy setting. They tried to keep their existence a secret from humans; while the Empire knew about the Skaven, and didn't like them, they also recognized that breaking the Masquerade in question would lead to open warfare, something they could ill-afford while fighting Chaos, beastmen, and vampires.
• In In Nomine, since divine or infernal interference can cause humans to not meet their Destiny or Fate, and Heaven is charged with trying to maximize the former and minimize the latter, and souls are the chief source of Essence for Hell, both sides have good reason to keep things on the low down.
• In Monster Hunter International the world governments believe that the more people who know about the supernatural the more powerful it becomes, and supernatural forces are always evil in this setting with many being potential world enders. As such the Monster Control Bureau and its international counterparts are often forced to intimidate, frame or even kill innocent witnesses to keep it up. The MCB director outright states in the second book that despite their best efforts he feels the truth will be exposed any day now in the wake of increasingly blatant supernatural events and modern communications.
• Urban Arcana is unusual in that the veil making shadowkind and supernatural acts look mundane is a force of nature rather than being an artificial construct. The more exposed someone is to unusual events, the weaker the effect becomes on them.

There are some obvious problems with most Masquerades. If you're curious, go look up TVTropes (or better yet, its uncensored fork allthetropes) article on the matter; they list quite a few usual flaws.