The name Azerbaijan is used today of both the sovereign Republic of Azerbaijan

Azerbaijan

Azerbaijan , officially the Republic of Azerbaijan is the largest country in the Caucasus region of Eurasia. Located at the crossroads of Western Asia and Eastern Europe, it is bounded by the Caspian Sea to the east, Russia to the north, Georgia to the northwest, Armenia to the west, and Iran to...

 and of the region of Azerbaijan

Azerbaijan (Iran)

Azerbaijan or Azarbaijan , also Iranian Azerbaijan, Persian Azarbaijan is a region in northwestern Iran. It is also historically known as Atropatene and Aturpatakan....

 in Iran

Iran

Iran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...

.

The name is a derivation from Atropatene

Atropatene

Atropatene was an ancient kingdom established and ruled under local ethnic Iranian dynasts first with "Darius" of Persia and later "Alexander" of Macedonia, starting in the 4th century BC and includes the territory of modern-day Iranian Azarbaijan and Iranian Kurdistan. Its capital was Gazaca...

, (Greek

Koine Greek

Koine Greek is the universal dialect of the Greek language spoken throughout post-Classical antiquity , developing from the Attic dialect, with admixture of elements especially from Ionic....

 Ατροπατήνη, mentioned in Strabo

Strabo

Strabo, also written Strabon was a Greek historian, geographer and philosopher.-Life:Strabo was born to an affluent family from Amaseia in Pontus , a city which he said was situated the approximate equivalent of 75 km from the Black Sea...

 11.523), the dominion of Atropates

Atropates

Atropates was a Persian nobleman who served Darius III, then Alexander III of Macedon, and eventually founded an independent kingdom and dynasty that was named after him...

, an Achaemenian satrap (governor), re-instated as ruler of Media

Medes

The MedesThe Medes...

 under Alexander of Macedonia.
The Middle Persian

Middle Persian

Middle Persian , indigenously known as "Pârsig" sometimes referred to as Pahlavi or Pehlevi, is the Middle Iranian language/ethnolect of Southwestern Iran that during Sassanid times became a prestige dialect and so came to be spoken in other regions as well. Middle Persian is classified as a...

 form of the name was Āturpātākān. It evolved further into early Modern Persian Ād̲h̲arbād̲h̲agān, Ād̲h̲arbāyagān, and finally Āzarbāyd̲j̲ān.

The etymology of the given name Atropates (Aτρoπάτης) itself is Old Persian Aturpat "protected by fire

Atar

Atar is the Zoroastrian concept of holy fire, sometimes described in abstract terms as "burning and unburning fire" or "visible and invisible fire" ....

".
The territory of what is now the Republic of Azerbaijan was known as Caucasian Albania

Caucasian Albania

Albania is a name for the historical region of the eastern Caucasus, that existed on the territory of present-day republic of...

 in antiquity, and later as Arran. The name of Azerbaijan Democratic Republic

Azerbaijan Democratic Republic

The Azerbaijan Democratic Republic was the first successful attempt to establish a democratic and secular republic in the Muslim world . The ADR was founded on May 28, 1918 after the collapse of the Russian Empire that began with the Russian Revolution of 1917 by Azerbaijani National Council in...

 was chosen in 1918 by the Musavat Party in the wake of the collapse of the Russian Revolution of 1917.