A chain is a reliable machine component, which transmits power by means of tensile forces, and is used primarily for power transmission and conveyance systems. The function and uses of chain are similar to a belt.

In set theory, a **chain** is a totally ordered subset of a partially ordered set. In algebraic topology, a chain is a formal combination of simplices.

A **chain** is a measurement of length equivalent to 22 yards (20.12 metres), which is one tenth of a furlong or one eightieth of a mile. It was used in surveying land in England and places under English influence, such as the United States, but is obsolete in many places. It is also called the **surveyor's chain** or **Gunter's chain** after 17th century mathematican and astronomer Edmund Gunter, who introduced it. The distance of 22 yards is the length of the pitch (from wicket to wicket) in cricket.

The **engineer's chain** or **Ramden's chain** of 100 feet (30.48m) was also used.

See also: U.S. customary units