Communio in liturgical use, is
(1) an anthem in the Roman and cognate missals, said by the celebrant after he has taken the ablutions. It is so called because it was originally appointed to be sung during the communion of the people, and was sung antiphonally after each verse of a psalm, which was continued till the priest gave the signal for the Gloria, when the communion of the people was ended. Afterwards the Communio was looked upon more as an act of thanksgiving to be said after the communion. It varies with the day.
(2) An anthem in the Mozarabic missal sung by the choir after the communion has taken place. There are only two forms: one used in Lent. the other during the rest of the year. — Smith, Dict. of Christ. Antiq. s.v.