It is possible to make an object or avatar “invisible” - for you only. It is useful in many situations, such as photography, for example, if an object is in your view and you do not wish it in the picture.
NOTE: There is a tutorial video on this topic here.
This is permanent derender; it will survive teleports and relogging. For more information, see the Asset Blacklist.
Temporary derendering will last until a view change is forced in some way.
Note that in Preferences → Move & View -> View, there is Re-render temporary derendered objects after teleport, which controls whether temporarily derendered obejcts will re-render after TP or not until after a relog.
See this page for documentation on Firestorm 4.7.9 (50527) and earlier.