Gusset

French gousset ("armpit, fob").

Full definition of gusset

(plural gussets)

A small piece of cloth inserted in a garment, for the purpose of strengthening some part or giving it a tapering enlargement.
A small piece of mail, providing some protection where two plates of armor are joined, usually at the elbows, under the shoulders, and behind the knees.
(machinery) A kind of bracket, or angular piece of iron, fastened in the angles of a structure to give strength or stiffness; especially, the part joining the barrel and the fire box of a locomotive boiler.
(heraldiccharge) An abatement or mark of dishonor in a coat of arms, resembling a gusset.
(roofing) A large flat metal piece wider than the valley to help prevent build-up at the base of the valley, either from debris or ice dam formations.