Intangibility is the psionic ability to pass ghost-like through solid objects. Possessors of this ability can render their entire bodies intangible, but also individual body parts such as arms. Possessors remain visible while using this ability. Possessors have not been shown to pass through floors.
It appears that when a possessor stops using his ability while still inside something, the material that was previously there either disappears or is displaced. e.g. when D.L. does this in Linderman's head, killing him.