Catechism of the Catholic Church
“The inexhaustible richness of this sacrament is expressed in the different names we give it. Each name evokes certain aspects of it. It is called:
Holy Mass (Missa), because the liturgy in which the mystery of salvation is accomplished concludes with the sending forth (missio) of the faithful, so that they may fulfill God’s will in their daily lives.“
Source: Catechism of the Catholic Church #1328-32