What is a "hoopskirt"?

A hoopskirt, also known as a crinoline or a hoop skirt, is a type of women's undergarment that has a frame of whalebone or steel hoops sewn into the fabric. The frame creates a bell-shaped silhouette, which became fashionable in the mid-19th century. Hoopskirts were initially worn under voluminous dresses, but later versions could be worn with lighter-weight garments. The popularity of hoopskirts waned by the early 20th century, but they have been used in period costumes and historical reenactments.