The Shadow World is the term generally used to refer to the hidden, supernatural part of the world that Shadowhunters and Downworlders share and is also being pervaded by demons. When referring only to the part inhabited by Downworlders, it is called the Downworld.
Beings born with the Sight naturally see past the glamours shielding the world from the mundane world. While the creatures and humans of the Shadow World are born with the Sight and see the Shadow World with ease, albeit needing a bit of training, "mundanes"—or most ordinary humans—rarely do, making most of them ignorant of and almost entirely separate from the world they technically share. It is mandated by the Clave and the Accords that everyone of the Shadow World must keep the existence of their supernatural societies a secret from mundanes.