Wiktionary
waist cincher
n. A type of corset that is used to pull in the waist.
Wikipedia
Waist cincher
A waist cincher (sometimes referred to as a Waspie) is a belt worn around the waist to make the wearer's waist physically smaller, or to create the illusion of being smaller.
Usage examples of "waist cincher".
She wore leather hip boots with long, sharp heels, shoulder-length leather gloves, a black waist cincher, and nothing else.