Fermentum is a practice of the Early Christian Church whereby bishops affirmed their communion with one another.
The custom of the fermentum was first practiced as early as 120 AD. A particle of the Eucharistic bread was carried by a minister of the Church from the bishop of one diocese to the bishop of another diocese. The receiving bishop would then consume the species at his next celebration of the Eucharist as a sign of the communion between the churches. The term fermentum was likely a reference to the Eucharist as the leaven of the Christian life, and the instrument by which Christians spread throughout the world were united in the one Body of Christ as a leaven to the world.
In the 2nd century, popes are known to have sent the Eucharist to other bishops as a pledge of unity of faith, this being the origin of the expression to be in communion with each other, already considered essential to Christianity in the 2nd century writings of St. Ignatius of Antioch and St. Irenaeus. On occasion, bishops would also send out fermenti to their priests.
John. D. Zizioulas (the Metropolitan of Pergamon) in his doctoral thesis (1964 University of Athens) takes the position that by the mid 3rd century the Bishops were exercising fermentum with the (local to the Metropolitan) parishes that did have a presiding Bishop in order to communicate/retain the unity of the Church under the Bishop. While this was due to the growth of the Christian church it is also due to extensive persecution of the Church, especially aimed at the Bishops.