A gag in the shape of a ring, used in BDSM activities. A ring gag prevents speech, because it forces the mouth open, but it does not prevent the wearer from making GAAARGH - AWG sounds.

The point of the ring gag is that the submissive wearer's mouth and tongue are, to a certain extent, available for whatever activities the dominant may think fit.

Ring gags are, like nose hooks, quite frequent in Japanese porn.