The modern ethnonym "Azerbaijani" or "Azeri" refers to the Turkic peoples of Iranian Azerbaijan and Republic of Azerbaijan.
They historically called themselves or were referred to by others as Muslims, Turks, Turkmens, Persians, Iranians, or Ajams – that is to say that religious identification prevailed over ethnic identification.
When the Southern Caucasus became part of the Russian Empire in the nineteenth century, the Russian authorities, who traditionally referred to all Turkic people as Tatars, defined Tatars living in the Transcaucasus region as Caucasian or Aderbeijanskie (Адербейджанские) Tatars in order to distinguish them from other Turkic groups.
The Russian Brockhaus and Efron Encyclopedic Dictionary, written in the 1890s, also referred to Tatars in Azerbaijan as Aderbeijans (адербейджаны), but noted that the term had not been widely adopted.
This ethnonym was also used by Joseph Deniker:
[The purely linguistic] grouping [does not] coincide with the somatological grouping: thus the Aderbeijani of the Caucasus and Persia, who speak a Turkic language, have the same physical type as the Hadjemi-Persians, who speak an Iranian tongue.

In Azerbaijani language publications, the expression "Azerbaijani nation" referring to those who were known as Tatars of the Caucasus first appeared in the newspaper Kashkul in 1880.
The modern ethnonym "Azerbaijani" or "Azeri" refers to the Turkic peoples of Iranian Azerbaijan and Republic of Azerbaijan. They historically called themselves or were referred to by others as Muslims, Turks, Turkmens, Persians, Iranians, or Ajams – that is to say that religious identification prevailed over ethnic identification. When the Southern Caucasus became part of the Russian Empire in the nineteenth century, the Russian authorities, who traditionally referred to all Turkic people as Tatars, defined Tatars living in the Transcaucasus region as Caucasian or Aderbeijanskie (Адербейджанские) Tatars in order to distinguish them from other Turkic groups. The Russian Brockhaus and Efron Encyclopedic Dictionary, written in the 1890s, also referred to Tatars in Azerbaijan as Aderbeijans (адербейджаны), but noted that the term had not been widely adopted. This ethnonym was also used by Joseph Deniker:
[The purely linguistic] grouping [does not] coincide with the somatological grouping: thus the Aderbeijani of the Caucasus and Persia, who speak a Turkic language, have the same physical type as the Hadjemi-Persians, who speak an Iranian tongue.
In Azerbaijani language publications, the expression "Azerbaijani nation" referring to those who were known as Tatars of the Caucasus first appeared in the newspaper Kashkul in 1880.
Azerbaijanis