Along with the Divine Office, the Mass forms the basis of Christian liturgy. It centers on the Eucharist, the celebration of Christ's sacrifice, which derived from his actions at the Last Supper and the agape, or love feast, of the early Church. The term stems from the dismissal at the end of the celebration, Ite, missa est ('the dismissal is here', or 'Go, [the congregation] is dismissed'). The texts for the performance of the mass, which include practices and formulae added over the centuries, were first contained in the Sacramentary and then in the Missal. The mass was attended daily by those in religious orders, the clergy, and, with varying frequency, by members of the laity.
Michelle Brown, Understanding Illuminated Manuscripts (Malibu, CA: J. Paul Getty Museum in association with the British Library, c1994).