Azerbaijani or Azeri or Azerbaijani Turkish (Azərbaycanca, Azərbaycan türkcəsi, Azərbaycan dili) is a language belonging to the Turkic language family, spoken in southwestern Asia by the Azerbaijani people, primarily in Azerbaijan and northwestern Iran. Azerbaijani is member of the Oghuz branch of the Turkic languages and is closely related to Turkish, Qashqai and Turkmen. Turkish and Azerbaijani are known to closely resemble each other, and the native speaker of one language is able to understand the other, though it is easier for a speaker of Azerbaijani to understand Turkish than the other way around.

Today′s Azerbaijani languages evolved from the Eastern Oghuz branch of Western (Oghuz) Turkic which spread to Southwestern Asia during medieval Turkic migrations, and has been heavily influenced by Persian.Arabic also influenced the language, but Arabic words were mainly transmitted through the intermediary of literary Persian.

Azerbaijani gradually supplanted the Iranian languages in what is now northern Iran (most notably the Tat, Azari, and Middle Persian varieties), and a variety of Caucasian languages in the Caucasus, particularly Udi. By the beginning of the 16th century, it had become the dominant language of the region, and was a spoken language in the court of the Safavid Empire. However, minorities in both Azerbaijan and Iran continue to speak the earlier Iranian languages to this day, and Middle- and Modern Persian loanwords are numerous in the Azerbaijani language.

The Azerbaijanis ( /ˌæzərbaɪˈdʒɑːni/; Azerbaijani: Azərbaycanlılar, Азәрбајҹанлылар, آذربایجانلیلار ) or Azarbaijanis are Turkic-speaking people living mainly in northwestern Iran and the Republic of Azerbaijan, as well as in the neighboring states, Georgia, Russia (Dagestan) and formerly Armenia. Also referred to as Azeris or Azaris (Azərilər, Азәриләр, آذریلر ) or Azerbaijani Turks (Azərbaycan türkləri), they also live in a wider area from the Caucasus to the Iranian plateau. The Azerbaijanis are predominantly Shi'a Muslim and have a mixed cultural heritage including Turkic, Iranic and Caucasian elements.

Following the Russo-Persian Wars of 1813 and 1828, the territories of the Qajar Persian Empire in the Caucasus were ceded to the Russian Empire and the treaties of Gulistan in 1813 and Turkmenchay in 1828 finalized the borders between Czarist Russia and Qajar Iran. The formation of the Azerbaijan Democratic Republic in 1918 established the territory of the Republic of Azerbaijan. Despite living on two sides of an international border, the Azeris form a single ethnic group. However, northerners and southerners differ due to nearly two centuries of separate social evolution in Iranian Azerbaijan and Russian/Soviet-influenced Azerbaijan. The Azerbaijani language unifies Azerbaijanis, and is mutually intelligible with Turkmen, Qashqai, Gagauz and Anatolian Turkish (including the dialects spoken by the Iraqi Turkmen), all of which belong to the Oghuz, or Western, group of Turkic languages.