A curtain is a piece of cloth or other material intended to block or obscure light, air drafts, or (in the case of a shower curtain), water.A curtain is also the movable screen or drape in a theatre that separates the stage from the auditorium or that serves as a backdrop/background.

Curtains are often hung on the inside of a building's windows to block the passage of light. For instance, at night to aid sleeping, or to stop light from escaping outside the building (stopping people outside from being able to see inside, often for privacy reasons). In this application, they are also known as "draperies". Curtains hung over a doorway are known as portières. Curtains come in a variety of shapes, materials, sizes, colours, and patterns. They often have their own sections within department stores, while some shops are completely dedicated to selling curtains.