DARBY, a borough of Delaware county, Pa., U.S.A., on the south-west border of Philadelphia, near the Delaware river; served by the Baltimore and Ohio, and the Pennsylvania railways. It is a residential suburb and has factories making cotton and woollen goods, yarn, motor boats, pearl buttons and water filters. Its population was 7,922 in 1920, and was 9,899 in 193o by the Federal census. Darby was settled by eight Friends, in 1682, and has one of the oldest libraries in the country. It was incorporated in 1853, but most of its development has taken place since 1900.