Adar, Aidor, a river in Belgorod Oblast, RSFSR, and Voroshilovgrad (since 1917, Lugansk) Oblast, Ukrainian SSR; the left tributary of the Severskii Donets. It is 264 km long, with a basin of 7,420 sq km. The river, which is unnavigable, flows along the southern edge of the Srednerusskaia elevation. It is used for irrigation. The city of Starobel’sk is on the Aidar River.