Objection 1. It seems that this sacrament is not suitably called by various names. For names should correspond with things. But this sacrament is one, as stated above (Article 2). Therefore, it ought not to be called by various names.
Objection 2. Further, a species is not properly denominated by what is common to the whole genus. But the Eucharist is a sacrament of the New Law; and it is common to all the sacraments for grace to be conferred by them, which the name "Eucharist" denotes, for it is the same thing as "good grace." Furthermore, all the sacraments bring us help on our journey through this present life, which is the notion conveyed by "Viaticum." Again something sacred is done in all the sacraments, which belongs to the notion of "Sacrifice"; and the faithful intercommunicate through all the sacraments, which this Greek word Synaxis and the Latin "Communio" express. Therefore, these names are not suitably adapted to this sacrament.
Objection 3. Further, a host [From Latin "hostia," a victim] seems to be the same as a sacrifice. Therefore, as it is not properly called a sacrifice, so neither is it properly termed a "Host."
On the contrary, is the use of these expressions by the faithful.
I answer that, This sacrament has a threefold significance. one with regard to the past, inasmuch as it is commemorative of our Lord's Passion, which was a true sacrifice, as stated above (Question 48, Article 3), and in this respect it is called a "Sacrifice."
With regard to the present it has another meaning, namely, that of Ecclesiastical unity, in which men are aggregated through this Sacrament; and in this respect it is called "Communion" or Synaxis. For Damascene says (De Fide Orth. iv) that "it is called Communion because we communicate with Christ through it, both because we partake of His flesh and Godhead, and because we communicate with and are united to one another through it."
With regard to the future it has a third meaning, inasmuch as this sacrament foreshadows the Divine fruition, which shall come to pass in heaven; and according to this it is called "Viaticum," because it supplies the way of winning thither. And in this respect it is also called the "Eucharist," that is, "good grace," because "the grace of God is life everlasting" (Romans 6:23); or because it really contains Christ, Who is "full of grace."
In Greek, moreover, it is called Metalepsis, i.e. "Assumption," because, as Damascene says (De Fide Orth. iv), "we thereby assume the Godhead of the Son."
Reply to Objection 1. There is nothing to hinder the same thing from being called by several names, according to its various properties or effects.
Reply to Objection 2. What is common to all the sacraments is attributed antonomastically to this one on account of its excellence.
Reply to Objection 3. This sacrament is called a "Sacrifice" inasmuch as it represents the Passion of Christ; but it is termed a "Host" inasmuch as it contains Christ, Who is "a host (Douay: 'sacrifice')... of sweetness" (Ephesians 5:2).
Source: Summa Theologica (New Advent)