Intangibility (sometimes called phasing) is the ability to pass through things physically. Characters with this ability are simply unaffected by attacks, as they pass through their bodies. This can also allow them to pass through walls and other physical objects. The exact mechanics of intangibility differs on the individual - some move so quickly that they phase through physical objects, and others are intangible due to being ghosts. Their intangibility may be constantly active or it may be something that they have to activate.
As with all example lists, this is not comprehensive.