INVIS

This spell causes the creature touched to vanish from sight and be
undetectable by normal vision or even infravision. Of course, the invisible
creature is not magically silenced, and certain other conditions can render
the creature detectable. Even allies cannot see the invisible creature or
his gear, unless those allies can normally see invisible things or employ
magic to do so. Items dropped or put down by the invisible creature become
visible. Items picked up disappear if tucked into the clothing or pouches
worn by the creature. This spell remains in effect until it is magically
broken or dispelled. Thus, the invisible being can open doors, talk, eat,
climb stairs, etc., but if he attacks, he immediately becomes visible,
although the invisibility enables him to attack first.
 
See also: Mass Invis, Detect Invis.