Catholicos (plural Catholicoi) is a title given to the head bishop of an autonomous region under the Patriarchate of Antioch in the ancient Syrian church. A Catholicos in all respects is equivalent to a Patriarch in powers, but, in precedence, defers to the Patriarch of Antioch. The word 'Catholicos' is a transliteration of the Greek καθολικός, pl. καθολικοί , meaning concerning the whole, universal or general.
There existed 4 Catholicoi in the ancient church[when?]:
Catholicos of the East in Persia/India
Catholicos of Etchmiadzin in Armenia
Catholicos of Iberia in Georgia
Catholicos of Caucasian Albania in Azerbaijan
All these 4 ancient catholicates were independent Autocephalous provinces under the Papal supervision of the Patriarch of Antioch prior to the creation of the Catholicos.
A Catholicate or Catholicosate (at times spelt as catholicossate) is the area of responsibility (territorial or otherwise) of the catholicos, considered a leader within any of the several churches of Eastern Christianity, especially those regarded as Oriental Orthodoxy.
In the Eastern Catholic Churches not of Byzantine Rite, Catholicos is equivalent to a Major Archbishop.

Origin of the title

The title arose in the Patrarichate of Antioch sometime after the division of jurisdiction of provinces among the three Churches in the First Council of Nicaea in 325 AD, that with canon six confirmed the traditional jurisdition of the Church of Antioch. History shows that catholicoi were created mainly to rule Eastern provinces that formed outside the Roman Empire which was part of patrarichate of Antioch as per the custom of ancient church.

The term 'Catholicos' first seems to have been applied to the churches in the Persian Empire, where it was believed that the holder was the successor of the Apostle Thomas. The first claim that the bishop of Selucia-Ctesiphon was superior to the other bishoprics and had (using a later term) patriarchal rights was made by Mar Papa bar Gaggai (or Aggai, c. 317-c. 329). In the 4th century this claim strengthened, and Mar Isaac (or Ishaq, 399-c.410), who organized the Council of Seleucia-Ctesiphon, used the title of bishop of Selucia-Ctesiphon, Catholicos and Head over the bishops of all the Orient. This line of Catholicos gave origin to the Church of the East and used the East Syrian Rite.

Catholicoi of the East

Autocephalous Churches of East Syrian Rite

The following are autocephalous Churches of East Syrian Rite that claim succession to the Catholicos of the East of Selucia-Ctesiphon of the Church of the East. These Churches were named Nestorian in the West (not a self-identification):
Catholicos of Assyrian Church of the East
Mar Dinkha IV is the current Catholicos-Patriarch of the Assyrian Church of the East.
Catholicos of Ancient Church of the East
Mar Addai II is the current Catholicos of the Ancient Church of the East.

Eastern Catholic Churches of East Syrian Rite

The following are Eastern Catholic Churches of East Syrian Rite that claim succession to the Catholicos of the East of Selucia-Ctesiphon of the Church of the East:
Catholicos of the Chaldean Catholic Church of the East
Mar Emmanuel III Delly is the current Catholicos-Patriarch of the Chaldean Catholic Church of the East (in full Communion with the Pope.)

Oriental Orthodox Church

In the 6th century, on initiative of Mar Jacob Baradaeus, a hierarchy of the Syrian Orthodox was erected in the present Iraq to serve the Christians who were not passed under the Catholicate of the Church of the East. The main See was placed in Tikrit and the rite used was the one of Antioch, i.e. the West Syrian Rite. The first head of this hierarchy was the Great Metropolitan Ahudemeh (559-575). From the 7th century the title used was Maphrian (or Maphryono). In the 12th century the See was moved to Mosul and from the 13th century the title became Catholicos of the East. After the massacres of Tamerlane, the Maphrian was forced to leave Persia and this title was used for the general vicar, with nominal right of succession, of the Patriarch of the Syrian Orthodox Church[4].
In the 17th century many Christians of the Kerala region in India decided to leave the Church of Malabar—the local Church that had been connected to the Church of the East and that from the second half of 16th century was under the control of the Portuguese—and to pass under the hierarchy of the Syriac Orthodox Church.[citation needed] They left the East Syrian Rite to adopt the West Syrian Rite. The title of Catholicos of the East for an Indian hierarchy was used from the 20th century.[citation needed] The following heads of the Orthodox Church claim succession to the Catholicos of the East.
Catholicos of Malankara Orthodox Syrian Church
Catholicos Baselios Marthoma Didymus I is the current Catholicos of the Malankara Orthodox Syrian Church.
Catholicos of Jacobite Syrian Orthodox Church
Baselios Thomas I is the current Catholicos of the Jacobite Syrian Orthodox church.

Armenian Churches

Armenian Apostolic Church

Likewise, the head of One Holy Universal Apostolic Orthodox Armenian Church also bore the title Catholicos.
The Catholicos of Etchmiadzin presides over the Supreme Spiritual Council of the Armenian Apostolic Church and is the head of the world's 7 million Armenian Orthodox Christians.
Catholicos of Etchmiadzin (Chief Shepherd and Pontiff to all Armenians dispersed throughout the world[5]) of the Armenian Apostolic Church,
Karekin II is the current Catholicos of the Armenian Apostolic Church.
There is a Catholicos residing in Antelias, Lebanon:
Catholicossate of the Great House of Cilicia (Since 1441).
His Holiness Aram I is the present Catholicos of the Great House of Cilicia
The primacy of honor of the Catholicossate of Etchmiadzin has always been recognized by the Catholicossate of Cilicia.
There once was a Catholicos in Akhtamar, a position that has since been abolished:
Catholicos of Akhtamar Island (1116-1895)

Armenian Catholic Church

Catholicos of the Armenian Catholic Church (in full Communion with the Pope).
Nerses Bedros XIX is the current Catholicos-Patriarch of the Armenian Catholic Church. His full title is officially "Patriarch Catholicos of the House of Cilicia"

Catholicos of Iberia

The title of catholicos is also used in the Georgian Church, whose head carries the title Catholicos-Patriarch of All Georgia.

Catholicos of Georgian Orthodox Church
Ilia II is the current Catholicos of the Georgian Orthodox Church.

Catholicos of Caucasian Albania

Historically, the title of Catholicos was also used by the chief bishop of Caucasian Albania. With the Islamic invasion this church deteriorated and the provinces came under the Catholicos of Etchmiadzin.