With contributions by pagankinktress. - Thanks!
Something which is put in the mouth to prevent speech or other noises.
For those who engage in BDSM play, the ball gag is probably most familiar; but some find these to be uncomfortable and rough on the jaw during prolonged use. These gags rate pretty high for the drool factor...which can be extremely sexy and humiliating all at once. If the sight of a drooling submissive turns you on, an especially fun game to engage in when your partner is bound in a ball gag is to pull on their lip until they begin to drool uncontrollably. Personally, I feel gags, and ball gags in particular, are less about eliciting silence from the wearer and more about placing the person in an intensely erotic position of humiliation.
Ball Gag Safety: http://www.ibondage.co.uk/ball_gag_safety.htm
For someone who has fairly long hair, you can pull locks of her hair forward and place into her mouth for a sensual, more personal type of gag.
Panties can be transformed into an intimate type of gag, especially after they've been worn for most of the day. If you are a female and are interested in driving your submissive insane with your scent, be sure to masturbate while wearing the panties you will gag your sub with. Then simply place the damp panties into the pet's mouth, making certain the crotch area is directly on the tongue. Then place some tape over the mouth and voila--a homemade panty gag that will leave your pet subdued, aroused and oh-so-deliciously vulnerable looking. If you are a male who enjoys wearing ladies underthings, you can perform this same exercise on yourself. Spend some time wearing your panties and be sure to get yourself off while you have them on. While they are still freshly decorated, immediately stuff the panties into your mouth and meditate on what vulnerability means to you.
There is something to be said for silencing your partner with a kiss. Especially if he or she is an excessively noisy one during play time. If you are concerned the neighbors might hear your pet's moans of pain and/or pleasure, plant a hard, open-mouthed kiss onto him and force his muffled moans directly into your mouth. This tends to be a most powerful exchange and subtle variation on breath play.