The Communion (Latin: communio; Ancient Greek: κοινωνικόν, koinonikon) is a refrain sung with psalm recitation during the distribution of the Eucharist in the Divine Liturgy or Mass. As chant it was connected with the ritual act of Christian communion.

According to Dimitri Conomos the koinonikon (κοινωνικόν), as it is sung as an elaborated communion chant during the Divine Liturgy, has derived from an early practice of psalm recitation similar to Western liturgies, when the Koinonikon served as a troparion. The oldest troparion which was used for communion, was "Γεύσασθε καὶ ἴδετε" ("O taste and see that the Lord is good", Ps. 33:9). It was supposed to symbolize the last supper celebrated on Maundy Thursday. During the 5th century, when the Divine Liturgy of the Presanctified Gifts had established and this communion chant became associated with it, the custom spread over the Lenten period, presumably with the recitation of different psalm sections (staseis).

In the early Asmatika (12th and 13th century), the choirbook of the cathedral rite, this koinonikon is classified as echos protos (transcribed as a—α′) according to the modal signatures of the Octoechos, but its archaic melos does not finish on the finalis and basis of this echos, but with the one (phthongos) of echos plagios devteros (transcribed as E—πλβ′).

In the cathedral rite of Constantinople the koinonikon as a troparion became so elaborated, that it was sung without psalm recitation.

Nevertheless, its text was usually a stichos taken from the psalter, like the Sunday Koinonikon of the Week Cycle Αἰνεῖτε τὸν κύριον ("Praise the Lord" Ps 148:1), which had already added as an Octoechos cycle in 13th-century Greek Asmatika, so that they could be performed according to the echos of the week. Within the weekly cycle each koinonikon was also specified to certain liturgical occasions such as the Wednesday koinonikon Ποτήριον σωτηρίου ("Cup of salvation" Ps 115:4) which was dedicated to feasts around the Theotokos or around martyres. Apart from the week cycle there was a repertory of 26 koinonika which developed as a calendaric cycle of immoveable and moveable feasts during the 9th century and they can be found in the books of the cathedral rite since the 12th century (psaltikon and asmatikon).

Since the 14th century, when a mixed rite had replaced the former tradition of the cathedral rite at Hagia Sophia, the old models have been elaborated in compositions of the Maistores like John Glykys, John Koukouzeles, and Manuel Chrysaphes.

The communion part of the Ambrosian Mass, as it had been celebrated in the cathedrals of Milan (called after the famous local bishop Ambrose), was composed around the Anaphora. It was opened by a litany called "Ter Kyrie", the Pater Noster, and the chant which preceded the Postcommunio, was called "Transitorium".

According to Isidore of Seville (Etymologiae, De ecclesiasticis officiis) and Pseudo-Germanus' Expositio Antiquae Liturgiae Gallicanae the communion chant of the Mass in the Gallican rite of France and the Visigothic rite of Spain was called Confractorium and probably connected with a ritual breaking (fraction) of sacramental bread.