Fermentum is a practice of the Early Christian Church whereby bishops affirmed their communion with one another, or with their own local subordinate priests.
The receiving bishop would then consume the species at his next celebration of the Eucharist as a sign of the communion between the churches.
[3] In the 2nd century, popes 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, and such communion already considered essential to Christianity in the 2nd-century writings of St. Ignatius of Antioch and St. Irenaeus.
Another usage, often termed sancta or also (confused with) fermentum, was to use a previously (locally) consecrated host to signify temporal continuity.
[2] At papal masses around 700 AD Klausen states: "Saying the salutation 'Pax Domini sit semper vobiscum' — [the peace of the Lord be always with you] — the Pope immersed the particle which came from the previous day's mass in the chalice and at the same time broke off a piece of bread which he had consecrated, and which was to serve as a fermentum at the next service".