The 4400: Invisibility

Invisibility is the state of an object that cannot be seen. Since objects can be seen by light in the visible spectrum from a source reflecting off their surfaces and hitting the viewer's eye, the most natural form of invisibility is an object that neither reflects nor absorbs light. In nature, this is known as transparency, and is seen in many naturally occurring materials.

Psychological Invisibility
It has been stated in some works of fiction that a telepath could cause something akin to invisibility by compelling others to look the other way or to focus on something to the exclusion of everything else.