A number of abilities enable you to turn yourself “invisible” - and this prevents you from being seen by most ordinary mortals.
The situation is a bit more complicated when supernaturalis and other magical creatures are involved.
Each creature has a “Detect Invisibility level”, and each invisible creature has an “Invisibility level”.
If a creature's current Detect Invisibility level is lower than your current Invisibility level, they can not usually see you, except if you are involved in combat. Whenever an invisible creature strikes (or is struck) in combat, its blur timer is set to 30 seconds. The creature then becomes partially visible as a blurred shape until their blur timer drops to 0, at which point they become fully invisible again.
If a creature cannot see you, it has a -50% penalty to its Defence against you, which changes to -25% if they see you as a blurred shape.
The Blind Fighting talent allows you to ignore the all Defence penalties. In addition, you are able to see partially visible people as if they were fully visible, and fully invisible people as if they were partially invisible, unless their invisibility is more than 5 ranks higher than your detection.
Invisibility does not affect how a blind creature perceives you (e.g. a character whose eyes are closed - see “help open”).
Both Invisibility and Detect Invisibility can be improved by spells, talents and powers.
Note that invisibility does not make you undetectable in other ways (i.e. people will still hear you say things, you can still be sensed entering the Nexus, etc).