Gusset

A gusset is a small piece of fabric, usually square or diamond-shaped, inserted into a seam to add room or strength, typically under the arm or at the crotch.

Where a garment is under strain, as at the underarm of a close sleeve, a gusset adds a small panel so the wearer can move without tearing the seams. Historic shirts and smocks, cut from rectangles to avoid waste, relied on gussets for fit.