"To count the terms used in theology as of primary importance, and to endeavor to trace out the hidden meaning in every phrase and in every syllable, is a characteristic wanting in those who are idle in the pursuit of true religion, but distinguishing all who get knowledge of the mark of our calling...The beginning of teaching is speech." --St. Basil the Great, On the Holy Spirit I.2
"I've often written here of the need to reclaim the Church's bipulmonary nature, that is, the fact that she has two lungs, as Pope Bl. John Paul II loved to say, the West and the East. I've recently realized that just having people mention the Eastern Church is not enough (though it is certainly a great start): people must also know what they're talking about in regards to Eastern Christianity. One of the greatest aspects that needs to be cleared up is simply the matter of terminology. Thus I here attempt to give a quick grammatical primer on terms that are often jumbled up by those sincerely wishing to do justice to the Church's bipulmonary nature. My goal is not to condemn those who inadvertently misuse language, but to instruct them and call them on to the correct use. Why does this matter? Because laziness with language, even if unintentional, feels somewhat like a lack of caring for the Eastern Church. I might be too sensitive, but I hope this primer will help either way. Any incorrect information given here is strictly due to my own faults.
Churches and Rites
Eastern Church = a very wide term, encompassing the Christian traditions rooted in the areas of (from roughly West to East) Eastern Europe, Greece, Egypt, Ethiopia, the Middle East (Syria, Lebanon, Antioch, Jerusalem, etc.), the Ukraine, Russia, and India. This includes both the Eastern Catholic Churches (Ruthenians (Byzantine Catholics), Ukrainian Catholics, Melkites, Maronites, etc.), the Eastern Orthodox Churches (Greek Orthodox, Russian Orthodox, Serbian Orthodox, etc.), and the Oriental Orthodox Churches (Assyrian Church of the East, Coptic Orthodox, Ethiopian Orthodox (Tewahedo), etc.).
Church = not the Church, but a particular Church; a Church is a Christian community that has retained apostolic succession and thus has valid Sacraments (Mysteries). Thus the Eastern Orthodox and Oriental Orthodox Churches are rightly called Churches (I'll get to the Eastern Catholic Churches' specific designation below), while those Christian denominations that resulted from the Protestant Reformation and other such movements are referred to as "ecclesial communities" because they lack valid Sacraments.
Oriental Orthodox Churches = those Eastern Churches that accept only the first three Ecumenical Councils (Nicea, Constantinople, Ephesus) and do not accept the authority of the Pope of Rome. They are also known as "non-Chalcedonian" Churches because they do not accept the Fourth Ecumenical Council, the Council of Chalcedon. They are also called "monophysite," "of one nature," (though at least some of them prefer the term "miaphysite," "of mixed nature") because they do not accept the doctrine of Chalcedon about the dual natures, human and divine, of Christ.
Eastern Orthodox Churches = those Eastern Churches that accept only the first seven Ecumenical Councils (Nicea, Constantinople, Ephesus, Chalcedon, II Constantinople, III Constantinople, II Nicea) and do not accept the authority of the Pope of Rome.
Eastern Catholic Churches = those Eastern Churches that accept all 21 Ecumenical Councils (see other sources for a list) and accept the authority of the Pope of Rome.
Sui iuris = self-governing. This is a term applied to Eastern Catholic Churches (and, in rare occasions, to some missions). The term (at least applied to Churches) seems to be an innovation of the 1990 Code of the Canons of Oriental Churches (CCEO), and all that this document says is that "A group of Christian faithful united by a hierarchy according to the norm of law which the supreme authority of the Church expressly or tacitly recognizes as sui iuris is called in this Code a Church sui iuris" (CCEO, can. 27). As a general idea, what sui iuris means is that each particular Church governs almost entirely on its own, with only certain powers being held solely by the Pope of Rome. (This is a somewhat poor description of sui iuris, but part of that is because there is little magesterial or other official ecclesial documentation on the term, from what I can find.)
Rite = "the liturgical, theological, spiritual and disciplinary patrimony, culture and circumstance of history of a distinct people, by which its own manner of living the faith is manifested in each Church sui iuris" (CCEO, can. 28 §1). There are five Eastern rites: Alexandrian, Antiochene (West Syriac), Armenian, Chaldean (East Syriac), and Constantinopolitan (Byzantine) (cf. CCEO, can. 28 §2). These rites (except for the Armenian rite) each are manifested in multiple Churches sui iuris: i.e. Greek Catholics, Melkites, and Ruthenian Catholics are all within the Byzantine Rite, and Maronites and Syro-Malankara Catholics are both within the Antiochene Rite. (As a side note, there are also multiple Western rites, such as Ambrosian and Carthusian, apart from the Roman Rite).
Byzantine Catholics = a term used in multiple ways. It is used to refer to 1. Catholics who belong to any of the Churches that use the Byzantine Rite, 2. any Eastern Catholics (which is most definitely an incorrect usage of the term), or 3. Catholics that are a part of the Ruthenian Catholic Church, now usually referred to as the Byzantine Catholic Church, under jurisdiction in the U.S. of the Byzantine Catholic Metropolia of Pittsburgh. The main confusion comes about due to the Ruthenian Church's shift in terminology to referring to themselves often as Byzantine Catholics to reduce their ethnic specificity. While a good thing in its expansion of the Eastern Christian tradition beyond its ethnic homelands, it does cause confusion in terminology.
Patriarch = A bishop who governs all other bishops and other faithful within his particular Church, but who is also given a special title of honor that gives them precedence of honor over all other bishops (after the Pope of Rome, of course). The tradition of patriarchs is rooted in the early Church with its pentarchy of great sees: Rome, Constantinople, Alexandria, Antioch, and Jerusalem, all of which had patriarchs (though the Patriarch of Rome was the Pope). The title of patriarch has now expanded to other sees, and in the Eastern Catholic Churches at the moment, there are six patriarchs. The title is one of a special honor on top of the governing power that any head of a particular Church has. (For more details on patriarch, see the long section in the CCEO, can. 55-150.)
Major Archbishop = A bishop who governs all other bishops and other faithful within his particular Church, but does not have the title of patriarch. Major archbishops have precedence of honor after patriarchs. (For more details, see CCEO, can. 151-154).
Metropolitan = A bishop who governs all other bishops within his see, called a metropolia. They can sometimes be the head of an entire particular Church (a metropolitan Church sui iuris), but they are not necessarily so. They are equivalent to an archbishop in the Roman Church, and the metropolia is equivalent to an archdiocese. (For details on metropolitan churches sui iuris, see CCEO, can. 155-173.)
Eparchy = The Eastern equivalent to the Roman diocese.
Liturgy = The Church's official public worship. The term includes more than just the celebration of the Sacraments: it also includes rituals such as the Divine Office (Liturgy of the Hours) and funeral rites, among many others. A liturgy is any service of official public worship performed by the Church.
Holy Mysteries = The common term in the Eastern Church for the Sacraments.
Eucharistic Liturgy = A liturgy that involves the consecration of the Eucharist. This includes not just the Mass, but liturgies such as the Divine Liturgy of St. John Chrysostom (Byzantine Rite) and the Holy Qurbana of Addai and Mari (Antiochene Rite). A service that only includes distribution of the Eucharist (such as the Liturgy of the Pre-Sanctified Gifts in the Byzantine Rite) would not, I think, merit this name.
Sacred Liturgy = Another term for Eucharistic Liturgy.
Divine Liturgy = The Eucharistic Liturgy of the Byzantine Rite, which comes in two forms, the Divine Liturgy of St. John Chrysostom and the Divine Liturgy of St. Basil the Great. (There is also a very infrequently-used form called the Divine Liturgy of St. James.) The Armenian Rite also uses the term for their Eucharistic Liturgy.
Mass = The Eucharistic Liturgy of the Roman Rite. It is also sometimes applied to other Eucharistic Liturgies of the Western Church, such as the Ambrosian Rite, Mozarabic Rite, Sarum Rite, and others. In addition, the term is used for the Eucharistic Liturgies of Western Rite Orthodox Churches and for the liturgies of some Protestant Churches. This is not the same as the Divine Liturgy: the terms should not be used interchangeably, for that shows, at least in some manner, a disrespect for the legitimate liturgical differences between the Roman Rite and other rites. Using the term Mass for the Eucharistic Liturgies of the Eastern Rites smacks of the "Latinization" of the past, which, in its harshest extremes, tried to make the Eastern Catholic Churches into Roman Catholics who spoke a different language. Please respect the legitimate liturgical diversity that enriches the Church (see the Catechism of the Catholic Church #1200-1209)."- reposted from Treasures of the Church with permission