A crinoline is a stiff or structured petticoat designed to hold out a skirt, popular at various times since the mid-19th century. Originally, crinoline described a stiff fabric made of horsehair ("crin") and cotton or linen which was used to make underskirts and as a dress lining. The term crin or crinoline continues to be applied to a nylon stiffening tape used for interfacing and lining hemlines in the 21st century.

A petticoat or underskirt is an article of clothing, a type of undergarment worn under a skirt or a dress. Its precise meaning varies over centuries and between countries.