chinstrap

noun

1. A strip of leather or material used to fasten a hat or helmet under the chin.

   “A bridle, the chin-strap or chain.”

   “Such a deficit was not likely long to escape notice: the regiment was halted in the midst of a manœuvre, and, in the pause that ensued, Nosey demanded, in a grunt of thunder, how, without a chinstrap, I could expect my cap to stick upon my head.”

   “I immediately spat my mouthpiece out and unbuckled my chinstrap, yanking my helmet off of my head in a determined flash.”

2. A form of beard that follows the jawline but does not cover the rest of the chin.
3. A band of colour below a bird's mouth, resembling the chinstrap of a helmet.

   “A miniature look-alike of the Canada Goose, with a partial white necklace instead of a white chinstrap.”

   “The dark slate-grey plumage contrasting strongly with white 'chinstrap' is shared only by Blue Eared-pheasant, but their ranges do not overlap.”