The Sight is the ability to see past glamour to a thing's true nature.
Most Shadowhunters are born with the Sight. Gaining the Sight and being able to see through glamours can be learned through training. Shadowhunters may have to concentrate for harder glamours.
There are mundanes who possess the Sight; others may be born with it, and others may acquire it at a later point in their lives, usually after exposure to elements of the Shadow World. Because of the inexplicable and strange things they see, many of these mundanes seek psychiatric treatment and take medications for hallucinations.