The Postcommunion, in Catholic liturgy, is the text said or sung on a reciting tone following the Communion of the Mass.
The Communion act finishes the essential Eucharistic service, and early Masses, as described by Justin Martyr, did not have anything afterward.
[1] A significant resemblance between the Roman Rite and that of the "Apostolic Constitutions" is that at Rome, too, there were formerly at every Mass two prayers of the same nature.
[1] In the "Gregorian Sacramentary" the second prayer, now called Super populum, occurs almost only from Septuagesima to Easter; the first, Ad complendum, continues throughout the year, but both have lost much of their original character.
This restriction apparently results from the shortening of the Mass (which explains many omissions and abbreviations) and the tendency of Lent to keep longer forms, such as more than two lessons.
Medieval commentators[2][1] explain this mystically; Honorius thinks the prayer to be a substitute for the Eastern blessed bread (antidoron).
The Postcommunion has lost much of its original character as a thanksgiving prayer and has absorbed the idea of the old Oratio ad populum.
[1] In contemporary Catholic usage, the postcommunion corresponds to the Prayer after Communion and is sung or recited audibly throughout by the celebrant.