From Burnley & Trowbridge:
Petticoat was the common term used for what we would call a skirt today! There were underpetticoats (like a half slip) and fashion petticoats. They were worn under gowns or as separate garments with a jacket or other upper garment. They were made of wool, linen, silk, cotton and some blends. This is a great project for your first 18th century garment with easy stitches and fit.