gusset

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A triangular insert, as in the seam of a garment, for added strength or expansion.
• noun A triangular metal bracket used to strengthen a joist.
• noun A piece of mail or plate armor protecting the joints in a suit of armor.

from The Century Dictionary.

• To make with a gusset; insert a gusset into, as a garment.
• noun In iron shipbuilding, a piece of plate of triangular form reinforcing on one side the junction of a part which meets another angularly, the gusset-plate being approximately normal to the line of the joint between the parts. See cut under bracket, 9 .
• noun A triangular plate or piece of cloth inserted or attached, to protect, strengthen, or fill out some part of a thing; a gore.
• noun An angular piece of iron or a kind of bracket fastened in the angles of a structure to give strength or stiffness. An angular piece of iron inserted in a boiler, tank, etc., where it changes from a cylindrical to a square form, as at the junction of the barrel and fire-box of a locomotive. A triangular piece of cloth inserted in a garment to strengthen or enlarge some part.
• noun In heraldry, same as gore, 7.

from the GNU version of the Collaborative International Dictionary of English.

• noun A small piece of cloth inserted in a garment, for the purpose of strengthening some part or giving it a tapering enlargement.
• noun Anything resembling a gusset in a garment.
• noun (Armor) A small piece of chain mail at the openings of the joints beneath the arms.
• noun (Mach.) A kind of bracket, or angular piece of iron, fastened in the angles of a structure to give strength or stiffness; esp., the part joining the barrel and the fire box of a locomotive boiler.
• noun (Her.) An abatement or mark of dishonor in a coat of arms, resembling a gusset.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A small piece of cloth inserted in a garment, for the purpose of strengthening some part or giving it a tapering enlargement.
• noun 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.
• noun machinery A kind of bracket, or angular piece of iron, fastened in the angles of a structure to give strength or stiffness; esp., the part joining the barrel and the fire box of a locomotive boiler.
• noun heraldry An abatement or mark of dishonor in a coat of arms, resembling a gusset.
• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a piece of chain mail covering a place unprotected by armor plate
• noun a piece of material used to strengthen or enlarge a garment
• noun a metal plate used to strengthen a joist