Phasing is the ability to move through objects by manipulating their own matter through frequencies and/or vibrations. This power has only been demonstrated by the unnatural beings known as the Dread Doctors, whose powers were based in frequencies, and the supernatural eemons known as the Oni, whose powers of Umbrakinesis allowed them to become one with the shadows and phase through objects.
The Oni, as creatures made up of darkness and shadow, often used their phasing power to melt into the shadows. The Oni were shown to be able to climb through the floor of Derek Hale's loft to where Lydia Martin was hiding so they could use their scanning power to determine if she was possessed by the Nogitsune. The Dread Doctors, as beings who bestowed upon themselves the power to control frequencies and vibrations through unnatural means, could vibrate their body's cells so that they became intangible, preventing them from being touched by any person or object.