A sensation of physical contact with an object that isn’t there. One type of tactile hallucination, the feeling that something is crawling on or under one’s skin, is also known as formication.
Example: The patient jumps up, whirls around, and looks confused. He explains that somebody just touched the back of his head, even though nobody else has been in the room with him for at least fifteen minutes.
I think this is a great description of tactile hallucinations: http://www.wisegeek.com/what-are-tactile-hallucinations.htm