Polychronion

The Polychronion is a solemn encomium chanted in the liturgy of the Eastern Orthodox and Byzantine Catholic churches. In the Roman rite, it is the equivalent of the liturgical acclamation Ad multos annos.
The Polychronion is chanted for the secular authorities, the church authorities, individuals on specific occasions, and the whole community of Orthodox Christians.

Forms

There are two forms of the Polychronion in the Slavic usage:

The more formal, which usually takes place at the end of the Divine Liturgy in which the deacon recites the names of the individuals to be commemorated, and the choir responds by chanting "Εἰς πολλὰ ἔτη", "Eis polla etē" three times. A version of this type may also be used to commemorate individuals on important days such as the day of their baptism, their name day or the anniversary of a clergyman's ordination.
In the less formal Polychronion the choir alone sings the commemorations, ending simply with "...may the Lord God preserve unto many years!", with no repetitions. This form is used at the end of Matins and Vespers.

For an Orthodox sovereign

The Polychronion as chanted for a sovereign is derived from the traditional Latin acclamation "Ad multos annos" accorded by the populace to Roman Emperors. The acclamation was continued in Byzantine times in partially hellenized form: "Immultos annos", before it was completely translated into the Greek form "Εἰς πολλὰ ἔτη":
Depending on circumstance, the names of other members of the reigning family can be added between the second and third lines, in the same form. Since there remain no Orthodox monarchies nowadays, the Polychronion is only chanted for ecclesiastical authorities, namely the Patriarch or diocesan bishop, in which case it is usually called the "Phēmē".