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.

According to the Oxford English Dictionary, in current British English, a petticoat is "a light loose undergarment ... hanging from the shoulders or waist". In modern American usage, "petticoat" refers only to a garment hanging from the waist. They are most often made of cotton, silk or tulle. Without petticoats, skirts of the 1850s would not have the volume they were known for. In historical contexts (16th to mid-19th centuries), petticoat refers to any separate skirt worn with a gown, bedgown, bodice or jacket; these petticoats are not, strictly speaking, underwear, as they were made to be seen. In both historical and modern contexts, petticoat refers to skirt-like undergarments worn for warmth or to give the skirt or dress the desired attractive shape.