A curtain is a piece of cloth intended to block or obscure light

Light

Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, or drafts

Air draft

Air draft is the distance from the surface of the water to the highest point on a vessel, similar to the "deep draft" of a vessel which is measured from the surface of the water to the deepest part of the hull below the surface, but "air draft" is expressed as a height, not a depth.The vessel's...

, or water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 in the case of a shower curtain. Curtains hung over a door

Door

A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside of a space....

way are known as portière

Portiere

A portière is a hanging placed over a door or over the doorless entrance to a room. Its name is derived from the French word for door, porte . From Asia it came to Europe at a remote date. It is known to have been in use in Europe in the 4th century , and was probably introduced much earlier...

s. Curtains are often hung on the inside of a building's window

Window

A window is a transparent or translucent opening in a wall or door that allows the passage of light and, if not closed or sealed, air and sound. Windows are usually glazed or covered in some other transparent or translucent material like float glass. Windows are held in place by frames, which...

 to block the passage of light, for instance at night

Night

Night or nighttime is the period of time when the sun is below the horizon. This occurs after dusk. The opposite of night is day...

 to aid sleep

Sleep

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

ing, or to stop light from escaping outside the building (stopping people outside from being able to see inside, often for privacy reasons).