In the early 1800s, under intense military and economic pressure, Iran was forced to cede its former lands in the Caucasus to Russia. The Russians initially called the region Eastern Transcaucasia. After the Bolshevik takeover, however, they divided the region and renamed it Georgia, Armenia, and Azerbaijan—a historically questionable name for what is now the Republic of Azerbaijan.
Iranian Azarbaijan is the land that originally bore the name Azarbaijan, and I will be using it to refer to the four Iranian provinces of West and East Azarbaijan, Ardabil, and Zanjan.