Koinonia is the Greek word for fellowship, but it means far more than a friendly gathering. Its root is the word for common or shared, and it describes a genuine sharing of life — a partnership, a communion. The early church devoted itself to koinonia alongside the apostles' teaching, the breaking of bread, and prayers, and shared even their possessions.
Koinonia is first of all vertical: John writes that our fellowship is with the Father, and with his Son Jesus Christ. Out of that shared life with God flows the horizontal sharing of believers with one another. It is the word behind the communion of the Lord's Supper (the koinonia of the body and blood of Christ) and behind the whole New Testament vision of the church as a family who belong to one another.
Koinonia in Scripture
That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.