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.

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, colors, and patterns. They often have their own sections within department stores, while some shops are completely dedicated to selling curtains.

Theater drapes and stage curtains serve a number of sound-absorbing and light-blocking functions, such as the front curtain that separates the stage from the auditorium, and other curtains and drapes that serve as backdrops or to obscure offstage areas.

Curtains vary according to cleanability, ultraviolet light deterioration, oil and dust retention, noise absorption, fire resistance, and life span. Curtains may be operated by hand, with cords, by press-button pads or remote-controlled computers. They are held out of the way of the window by means of curtain tie-backs. Measuring curtain sizes needed for each window varies greatly according to the type of curtain needed, window size, and type and weight of curtain.

Curtains are a form of window decor and complete the overall appearance of the interior of the house. Curtains help control the ambiance and flow of natural light into the room. The effect of drapery or curtains is best seen in daylight, and with proper indoor light positioning, can look attractive even at night.

From evidence found in excavation sites at Olynthus, Pompeii and Herculaneum, portieres, a curtain hung over a doorway, appear to have been used as room dividers in classic antiquity. Mosaics from the 2nd to 6th century show curtains suspended from rods spanning arches.

In England, curtains began to replace wooden shutters towards the end of the 16th century.^{[better source needed]} In medieval England, the earliest form of window treatments were leather panels threaded onto iron rods. These were eventually replaced with woven wool panels. During the reign of Elizabeth I, ornately decorated Italian Renaissance fabrics, including brocades, velvets, and damask, began to be imported. These ornate fabrics, as well as fabrics decorated with crewel embroidery, were used in curtains during the English Elizabethan and Jacobean periods. Solid wood shutters were used during cold seasons.

In the Fatimid Caliphate, a curtain known as a sitr ("veil") was used to conceal the caliph at the beginning of an audience session (majlis). A servant known as the ṣāḥib al-sitr (or muṭawallī al-sitr) would then pull it back to reveal the caliph seated on his throne. The sahib al-sitr was also combined with the duties of chamberlain, master of ceremonies, and bearer of the caliph's sword, and was often chosen from among mamluks from saqaliba backgrounds.