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:
The Holy and Divine Liturgy, because the Church’s whole liturgy finds its center and most intense expression in the celebration of this sacrament; in the same sense we also call its celebration the Sacred Mysteries. We speak of the Most Blessed Sacrament because it is the Sacrament of sacraments. the Eucharistic species reserved in the tabernacle are designated by this same name.”
Source: Catechism of the Catholic Church #1328-30