A Katholikon or Catholicon is the major temple

Temple

A temple is a structure reserved for religious or spiritual activities, such as prayer and sacrifice, or analogous rites. A templum constituted a sacred precinct as defined by a priest, or augur. It has the same root as the word "template," a plan in preparation of the building that was marked out...

 (church building) of a monastery

Monastery

Monastery denotes the building, or complex of buildings, that houses a room reserved for prayer as well as the domestic quarters and workplace of monastics, whether monks or nuns, and whether living in community or alone .Monasteries may vary greatly in size – a small dwelling accommodating only...

, or diocese

Diocese

A diocese is the district or see under the supervision of a bishop. It is divided into parishes.An archdiocese is more significant than a diocese. An archdiocese is presided over by an archbishop whose see may have or had importance due to size or historical significance...

 in the Eastern Orthodox Church

Eastern Orthodox Church

The Orthodox Church, officially called the Orthodox Catholic Church and commonly referred to as the Eastern Orthodox Church, is the second largest Christian denomination in the world, with an estimated 300 million adherents mainly in the countries of Belarus, Bulgaria, Cyprus, Georgia, Greece,...

. The name derives from the fact that it is (usually) the largest temple where all gather together to celebrate the major feast days of the liturgical year

Liturgical year

The liturgical year, also known as the church year, consists of the cycle of liturgical seasons in Christian churches which determines when feast days, including celebrations of saints, are to be observed, and which portions of Scripture are to be read. Distinct liturgical colours may appear in...

. At other times, the smaller temples or chapel

Chapel

A chapel is a building used by Christians as a place of fellowship and worship. It may be part of a larger structure or complex, such as a church, college, hospital, palace, prison or funeral home, located on board a military or commercial ship, or it may be an entirely free-standing building,...

s would be used. In Russia, it is common for a katholikon to have a smaller church in the basement which can be more easily heated in the winter.

A katholikon may have special architectural features in it, such as a kathedra (episcopal throne

Throne

A throne is the official chair or seat upon which a monarch is seated on state or ceremonial occasions. "Throne" in an abstract sense can also refer to the monarchy or the Crown itself, an instance of metonymy, and is also used in many expressions such as "the power behind the...

), or both an esonarthex (inner-narthex

Narthex

The narthex of a church is the entrance or lobby area, located at the end of the nave, at the far end from the church's main altar. Traditionally the narthex was a part of the church building, but was not considered part of the church proper...

) and exonarthex (outer narthex), used for special services such as during Holy Week

Holy Week

Holy Week in Christianity is the last week of Lent and the week before Easter...

 or the Paschal vigil.

The word katholikon is often translated into English as cathedral

Cathedral

A cathedral is a Christian church that contains the seat of a bishop...

; however, unlike in Western Christianity

Western Christianity

Western Christianity is a term used to include the Latin Rite of the Catholic Church and groups historically derivative thereof, including the churches of the Anglican and Protestant traditions, which share common attributes that can be traced back to their medieval heritage...

 an Eastern Orthodox diocese may have more than one cathedral. In addition to a bishop's primary cathedral, other important churches may be elevated to the dignity of cathedral, without the bishop actually being resident there. The rector

Rector

The word rector has a number of different meanings; it is widely used to refer to an academic, religious or political administrator...

 of such churches will usually be an Archpriest

Archpriest

An archpriest is a priest with supervisory duties over a number of parishes. The term is most often used in Eastern Orthodoxy and Eastern Catholic Churches, although it may be used in the Latin rite of the Roman Catholic Church instead of dean or vicar forane.In the 16th and 17th centuries, during...

 or Protopresbyter.