Excerpts from "The Orthodox Faith" by Protopresbyter Thomas Hopko:
The Holy Eucharist is called the “sacrament of sacraments” in the Orthodox tradition. It is also called the “sacrament of the Church.” The eucharist is the center of the Church’s life. Everything in the Church leads to the eucharist, and all things flow from it. It is the completion of all of the Church’s sacraments—the source and the goal of all of the Church’s doctrines and institutions.
As a word, the term eucharist means thanksgiving. This name is given to the sacred meal-not only to the elements of bread and wine, but to the whole act of gathering, praying, reading the Holy Scriptures and proclaiming God’s Word, remembering Christ and eating and drinking his Body and Blood in communion with him and with God the Father, by the Holy Spirit. The word eucharist is used because the all-embracing meaning of the Lord’s Banquet is that of thanksgiving to God in Christ and the Holy Spirit for all that he has done in making, saving and glorifying the world.
The sacrament of the eucharist is also called holy communion since it is the mystical communion of men with God, with each other, and with all men and all things in him through Christ and the Spirit. The eucharistic liturgy is celebrated in the Church every Sunday, the Day of the Lord, as well as on feast days. Except in monasteries, it is rarely celebrated daily. Holy Communion is forbidden to all Orthodox Christians on the week days of Great Lent except in the special communion of the Liturgy of the Pre-sanctified Gifts because of its joyful and resurrectional character. The eucharist is always given to all members of the Church, including infants who are baptized and confirmed. It is always given in both forms—bread and wine. It is strictly understood as being the real presence of Christ, His true Body and Blood mystically present in the bread and wine which are offered to the Father in his name and consecrated by the divine Spirit of God.
The mystery of the holy eucharist defies analysis and explanation in purely rational and logical terms. For the eucharist—and Christ Himself—is indeed a mystery of the Kingdom of Heaven which, as Jesus has told us, is “not of this world.” The eucharist—because it belongs to God’s Kingdom—is truly free from the earth-born “logic” of fallen humanity.