a young person (a boy or a girl) who is invited by the parish to assist the priest at the Eucharist (Mass) and other liturgies of the Church

An altar server is a lay assistant to a member of the clergy during a religious service. Altar servers attend to supporting tasks at the altar such as fetching and carrying, ringing a bell etc. An altar server usually has no formal training and holds no formal religious position; however, in some regions and religions, altar servers are expected to be recognizable through behavior and relation to others.