Koinonia  is a transliterated form of the Greek word , which refers to communion or fellowship, joint participation, the share which one has in anything, a gift jointly contributed, a collection, a contribution, etc. It identifies the idealized state of fellowship and unity that should exist within the Christian church, the Body of Christ.
The essential meaning of the koinonia embraces concepts conveyed in the English terms community, communion, joint participation, sharing and intimacy. Koinonia can therefore refer in some contexts to a jointly contributed gift. The word appears 19 times in most editions of the Greek New Testament. In the New American Standard Bible, it is translated "fellowship" twelve times, "sharing" three times, and "participation" and "contribution" twice each.
Koinonia appears nowhere in the ancient Greek translation of the Old Testament known as the Septuagint. It sometimes has a secular significance, as in the case of the partnership between fishermen spoken of in Luke 5:7-10. As a noun, or in its adjectival or verbal forms, it is found in 43 verses of the New Testament. In addition, the noun is found in some manuscripts (used for producing the English translation known as the King James Version, but not for more recent translations) in Ephesians 3:9.
In the New Testament the word is applied, according to the context, to communion, sharing or fellowship with:
Of these usages, Bromiley's International Standard Bible Encyclopedia selects as especially significant the following meanings:
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The Eucharist is the sacrament of communion with one another in the one body of Christ. This was the full meaning of eucharistic koinonia in the early Catholic Church. St. Thomas Aquinas wrote, "the Eucharist is the sacrament of the unity of the Church, which results from the fact that many are one in Christ."
If the relationship between the churches is complete, involving fullness of "those bonds of communion - faith, sacraments and pastoral governance - that permit the Faithful to receive the life of grace within the Church", it is called full communion. However, the term "full communion" is frequently used in a broader sense, to refer instead to a relationship between Christian churches that are not united, but have only entered into an arrangement whereby members of each church have certain rights within the other.
If a church recognizes that another church, with which it lacks bonds of pastoral governance, shares with it some of the beliefs and essential practices of Christianity, it may speak of "partial communion" between it and the other church.
The communion of saints is the relationship that, according to the belief of Christians, exists between them as people made holy by their link with Christ. That this relationship extends not only to those still in earthly life, but also to those who have gone past death to be "away from the body and at home with the Lord" (2 Corinthians 5:8) is a general belief among Christians. Their communion is believed to be "a vital fellowship between all the redeemed, on earth and in the next life, that is based on the common possession of the divine life of grace that comes to us through the risen Christ".
Since the word rendered in English as "saints" can mean not only "holy people" but also "holy things", "communion of saints" also applies to the sharing by members of the church in the holy things of faith, sacraments (especially the Eucharist), and the other spiritual graces and gifts that they have in common.
The term "communion" is applied to sharing in the Eucharist by partaking of the consecrated bread and wine, an action seen as entering into a particularly close relationship with Christ. Sometimes the term is applied not only to this partaking but to the whole of the rite or to the consecrated elements.