In today’s usage, a girdle is most commonly known as a support garment worn by women to give the overall appearance of a flat tummy and more shapely buttocks. It is often called support underwear, or support panties. Many nylons incorporate support elements or a control top, like strong elastic around the stomach and the hips, which make wearing a separate support garment superfluous and less popular.
Actually, earlier girdles, prior to the advent of nylons, generally did not have a panty component. They were a tight garment, made to flatten the tummy, and often had straps that attached to the top of thigh high stockings and held them up.
The word actually predates this sort of garment, and in Ancient Greece and Rome, it was used for a thin belt worn around the waist, most often by men. Both women and men might wear such a belt during the Middle Ages, and one is famously mentioned in the West-Midland English Poem, “Sir Gawain and The Green Knight.” The wife of the Green Knight offers Sir Gawain the red girdle wrapped round her waist, which is supposed to protect him from injury.
Men or women may still wear certain types of girdles as a form of back support. Since the term has come to be associated more with women than men, however, ones of this type might simply be called a back supporting belt.
The girdle replaced the more restrictive corset worn by women. It still provided some waist control and tummy control, but did not cinch the waist in as tightly as corseting did. This resulted in a more comfortable garment, though many women still find the pressure from shape underwear too tight for comfort.
Some shapewear with an underwear component do reach above the waist, and they may start just below the breasts to give the appearance of a smaller waist. Others are more like teddies, and they incorporate a bra with underwire and underpants in one garment. These may be harder to fit on women who have a greater distribution of weight on top than on bottom, or vice versa.
The modern undergarment has benefited from the development of spandex, which is lightweight and can hold the stomach or buttocks in with relative ease. Wearing too small a girdle, however, generally does not accomplish much. Excess skin or fat is usually merely pushed above or below the garment.
Shapewear garments may provide a little support, and may be especially helpful in close and formfitting clothing. Many women prefer a natural shape and more comfortable underwear, however, so the traditional girdle is not as popular as it once was.