This article is about the effect; for the spell, see Invisibility
Invisible creatures cannot be seen by non-magical vision. They cannot be detected with the spot or (as of v1.23) listen skills. Invisibility inherently grants 50% concealment, and anyone attacked by an invisible creature is considered flat-footed. Furthermore, invisible creatures receive a +2 attack bonus unless negated by the target's blind fighting. Attacking, or performing any hostile act, causes invisibility to be lost, though. This usually includes hostilities performed by a lingering spell (cast by the subject before becoming invisible), but does not include summoning a creature nor hostile actions taken by a summoned creature.
The types of magical vision that can see invisible creatures are Blindsight, see invisibility and true seeing. Recipients of these spells may also bypass the concealment inherent to invisibility (but not other types of concealment; in particular, greater invisibility has a separate 50% concealment effect that is not negated).
Attacking a creature that cannot be seen gives a -4 penalty to attack rolls. In particular, this usually applies when attacking an invisible creature.
Since some creatures can detect or even see invisible creatures, it can be helpful to use stealth mode even when invisible.
Note: Uncanny Dodge will prevent a defender from losing their dexterity and dodge bonuses to AC against invisible attackers.
See also: Etherealness