The Catechism points out that the Holy Eucharist is both a sacrifice and a sacrament.

As a sacrifice the Holy Eucharist is the Mass. The Mass is that divine action in which Jesus, through the agency of the human priest, changes the bread and wine into His own Body and Blood and continues through time the offering which He made to God on Calvary—the offering of Himself for mankind.

It is at the consecration of the Mass that the sacrament of the Holy Eucharist comes into being. It is then that Jesus becomes present under the appearance of bread and wine. As long as the appearances of bread and wine remain, Jesus remains present and the sacrament of the Holy Eucharist continues to there exist.

The act by which we receive the Holy Eucharist is called Holy Communion.

We might say that the Mass is the "making" of the Holy Eucharist and Holy Communion is the receiving of the Holy Eucharist. In between the two, the sacrament of the Holy Eucharist continues to exist (as in the tabernacle) whether we receive it or not.