Masquerade

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.

There are some obvious problems with most Masquerades. If you're curious, go look up TVTropes article on the matter; they list quite a few usual flaws.