A bondgage hood (also called a gimp mask) is a fetishistic hood. It may be made from rubber, PVC, or leather. It is designed to completely cover the head, and is thus accommodated with an opening at the back to extend the neck hole for adorning - which may be done up with a zip or cord. Other openings include 2 eye holes (sometimes these can be covered with a blindfold), optional nostril openings, and a mouth opening (which is often closed with a zip or a gag).
An Isolation hood is a more extreme version of a bondage hood; it is design to disorientate the wearer by keeping them in total darkness and making it very difficult to hear anything. Most isolation hood also incorporate gags to enhance the sensation of loss of control and helpless felt by the victim.
Note that this is potentially extremely dangerous, as blocking all access to air will result in rapid asphyxiation. The safety warnings regarding gags also apply.
It is notably used in BDSM, its function being to objectify the wearer as well as providing sensory deprivation, and thus reduce them to the status of a sexual toy, rather than a sexual partner, as part of a scene. It can also serve the function of disguising their identity when desired, for example at play parties.