The Holy Eucharist is a sacrament and a sacrifice. In the Holy Eucharist, under the appearances of bread and wine, the Lord Christ is contained, offered, and received.
In the Holy Eucharist, instituted by Christ at the Last Supper in the presence of the apostles, the entire substance of the bread and the entire substance of the wine are changed into His Body and Blood. The whole Christ is really, truly, and substantially present in the Holy Eucharist.
For more information about the sacrament, please click on the EWTN link below: