A person who assists the priest in giving either Holy Communion or the precious blood (chalice) at Saturday, Sunday or weekday Masses.
When the size of the congregation or the incapacity of the bishop, priest, or deacon requires it, the celebrant may be assisted by other bishops, priests, or deacons. If such ordinary ministers of Holy Communion are not present, "the priest may call upon extraordinary ministers to assist him, i.e., formally instituted acolytes or even some of the faithful who have been commissioned according to the prescribed rite. In case of necessity, the priest may also commission suitable members of the faithful for the occasion."
Extraordinary ministers of Holy Communion should receive sufficient spiritual, theological, and practical preparation to fulfill their role with knowledge and reverence. When recourse is had to Extraordinary Ministers of Holy Communion, especially in the distribution of Holy Communion under both kinds, their number should not be increased beyond what is requiered for the orderly and reverent distribution of the Body and Blood of the Lord. In all matters such Extraordinary Ministers of Holy Communion should follow the guidance of the diocesan bishop. Holy Communion under both Kinds (Species) from the first days of the Church's celebration of the Eucharist, Holy Communion consisted of the reception of both species in fulfillment of the Lord's command to "take and eat...take and drink." The practice of Holy Communion under both kinds at Mass continued until the late 11th century, when the custom of distributing the Eucharist to the faithful under the form of bread alone began to grow.
By the 12th century, theologians speak of Communion under one kind as a "custom" of the Church. The practice spread until the Council of Constance in 1415 decreed that Holy Communion under the form of bread alone would be distributed to the faithful. In 1963, the Fathers of the Second Vatican Council authorized the extension of the faculty for Holy Communion under both kinds...at the Bishop's discretion. (See Norms 18, 19 & 20) Holy Communion has a more complete form as a sign when it is received under both kinds. For in this manner of reception a fuller sign of the Eucharistic banquet shines forth. Clearly, there are some pastoral circumstances that require Eucharistic sharing in one species. (Norms 15) The Church has always taught the doctrine of concomitance, by which we know that under each species alone, the whole Christ is sacramentally present and we "receive all the fruit of Eucharistic grace." (GIRM 281 & CCC 1390)