Gusset
Gusset is a panel, either triangular or diamond in shape, that is inserted into a garment to help shape and reinforce key points, like the underarms or crotch. You get gussets in modern tights and pantyhose — they add breadth and breathe to the crotch seam. It’s from this panel that that the term ‘don’t bust a gusset’ (don’t rip your pants) originates.