A gusset is the panel of material — leather, canvas, or fabric — that forms the sides and/or base of a bag, creating the three-dimensional structure. On a flat clutch, there's no gusset — the bag has no depth. On a roomy tote or shoulder bag, the gusset provides the full volume of the interior.
Gusset width is the primary determinant of bag capacity. A bag described as 'with a 10cm gusset' will hold considerably more than the same bag with a 5cm gusset, even if the external dimensions look similar. When assessing a bag's practical use, checking the gusset measurement is more informative than height and width alone.