Name and etymology
The city was originally known in Armenian as Karno K'aghak' (Armenian: Կարնո քաղաք), meaning city of Karin, to distinguish it from the district of Karin (Կարին). After the Arab conquest of Armenia, the city was known to the Arabs as Kālīkalā (which was adopted from the original Armenian name).
During Roman times, Erzurum was named Theodosiopolis (Latin: Theodosiopolis, Greek: Θεοδοσιούπολις), or – in Armenia or – in Cappadocia to distinguish is from several namesakes. It got its present name after its conquest by the Seljuks following the Battle of Manzikert in 1071.
A neighboring commercial city named Artze (Arcn, Arzan; Armenian: Արծն) was heavily sacked by the Seljuk Turks in 1048/49. Its Armenian, Syrian, and other Christian inhabitants moved to Theodosiopolis, which they began calling "Artsn Rum" (meaning Artze of the Rûm, i.e., Romans) to distinguish it from their former residence. After the Arab conquest of Armenia, the city was known to the Arabs as Kālīkalā (which was adopted from the original Armenian name Karno K'aghak' (Armenian: Կարնո քաղաք), meaning "Karin City", to distinguish it from the district of Karin (Կարին). Some older sources derive the name Erzurum from the Arabic Arḍ ar-Rūm (Arabic: ارض الروم) 'land of the Rûm'.
In the words of Parvaneh Pourshariati / Encyclopædia Iranica:
In fact, the powerful noble family of the Kamsarakan in Armenia traced their genealogy to the Iranian Kārin Pahlav family of the Arsacid period, and specifically to one Pērōzmat (only attested by Movsēs Xorenacʿi, p. 219). The Armenian Kārins, the Kamsarakan, remained a powerful dynastic family in the region, directly involved in the history of the Byzantines and the Sasanians, and in Armenian political sphere up to the 14th century, carrying the surname of Pahlavuni, in commemoration of their origins. They lent their name to important localities, so that ancient Theodosiopolis was named Kārin, before the name was changed to Erzurum in later centuries.
During the Georgian rule city was called as Karnu-kalaki (Georgian: კარნუ-ქალაქი).