The toponym "Azerbaijan" was historically used to refer to the region located south of the Aras River- today known as Iranian Azerbaijan, located in northwestern Iran.[1][2] Historians and geographers usually referred to the region north of the Aras River as Arran,[3][4][5] but the name "Azerbaijan" has also been extended to this area as well. On May 28, 1918, following the collapse of the Russian Empire, the Azerbaijan Democratic Republic was proclaimed to the north of the Aras.
AZERBAIJAN (Āḏarbāy[e]jān), historical region of northwestern Iran, east of Lake Urmia, since the Achaemenid era. The name Azerbaijan was also adopted for Arrān, historically an Iranian region, by anti-Russian separatist forces of the area when, on 26 May 1918, they declared its independence and called it the Democratic Republic of Azerbaijan. To allay Iranian concerns, the Azerbaijan government used the term "Caucasian Azerbaijan" in the documents for circulation abroad. This new entity consisted of the former Iranian Khanates of Arrān, including Karabagh, Baku, Shirvan, Ganja, Talysh (Ṭāleš), Derbent (Darband), Kuba, and Nakhichevan (Naḵjavān), which had been annexed to Russia by the treaties of Golestān (1813) and Torkamānčāy (1828) under the rubric of Eastern Transcaucasia.
Prior to 1918, the term "Azerbaijan" applied only to the Iranian province of Azarbayjan.