Aarau, a town of Switzerland, capital of the canton of Aargau, on the Aar; pop. in 1870, 5,449. The town is well built, and is celebrated for its manufactories of mathematical instruments. In August, 1712, a peace was concluded at Aarau between the cantons of Bern and Unterwalden. During the time of the Helvetic republic (1798) Aarau was the seat of the central government.