Objection 1. It would seem that "Word" is not the proper name of the Son. For the Son is a subsisting person in God. But word does not signify a subsisting thing, as appears in ourselves. Therefore word cannot be the proper name of the person of the Son.
Objection 2. Further, the word proceeds from the speaker by being uttered. Therefore if the Son is properly the word, He proceeds from the Father, by way only of utterance; which is the heresy of Valentine; as appears from Augustine (De Haeres. xi).
Objection 3. Further, every proper name of a person signifies some property of that person. Therefore, if the Word is the Son's proper name, it signifies some property of His; and thus there will be several more properties in God than those above mentioned.
Objection 4. Further, whoever understands conceives a word in the act of understanding. But the Son understands. Therefore some word belongs to the Son; and consequently to be Word is not proper to the Son.
Objection 5. Further, it is said of the Son (Hebrews 1:3): "Bearing all things by the word of His power;" whence Basil infers (Cont. Eunom. v, 11) that the Holy Ghost is the Son's Word. Therefore to be Word is not proper to the Son.
On the contrary, Augustine says (De Trin. vi, 11): "By Word we understand the Son alone."
I answer that, "Word," said of God in its proper sense, is used personally, and is the proper name of the person of the Son. For it signifies an emanation of the intellect: and the person Who proceeds in God, by way of emanation of the intellect, is called the Son; and this procession is called generation, as we have shown above (Question 27, Article 2). Hence it follows that the Son alone is properly called Word in God.
Reply to Objection 1. "To be" and "to understand" are not the same in us. Hence that which in us has intellectual being, does not belong to our nature. But in God "to be" and "to understand" are one and the same: hence the Word of God is not an accident in Him, or an effect of His; but belongs to His very nature. And therefore it must needs be something subsistent; for whatever is in the nature of God subsists; and so Damascene says (De Fide Orth. i, 18) that "the Word of God is substantial and has a hypostatic being; but other words [as our own] are activities if the soul."
Reply to Objection 2. The error of Valentine was condemned, not as the Arians pretended, because he asserted that the Son was born by being uttered, as Hilary relates (De Trin. vi); but on account of the different mode of utterance proposed by its author, as appears from Augustine (De Haeres. xi).
Reply to Objection 3. In the term "Word" the same property is comprised as in the name Son. Hence Augustine says (De Trin. vii, 11): "Word and Son express the same." For the Son's nativity, which is His personal property, is signified by different names, which are attributed to the Son to express His perfection in various ways. To show that He is of the same nature as the Father, He is called the Son; to show that He is co-eternal, He is called the Splendor; to show that He is altogether like, He is called the Image; to show that He is begotten immaterially, He is called the Word. All these truths cannot be expressed by only one name.
Reply to Objection 4. To be intelligent belongs to the Son, in the same way as it belongs to Him to be God, since to understand is said of God essentially, as stated above (14, 2,4). Now the Son is God begotten, and not God begetting; and hence He is intelligent, not as producing a Word, but as the Word proceeding; forasmuch as in God the Word proceeding does not differ really from the divine intellect, but is distinguished from the principle of the Word only by relation.
Reply to Objection 5. When it is said of the Son, "Bearing all things by the word of His power"; "word" is taken figuratively for the effect of the Word. Hence a gloss says that "word" is here taken to mean command; inasmuch as by the effect of the power of the Word, things are kept in being, as also by the effect of the power of the Word things are brought into being. Basil speaks widely and figuratively in applying Word to the Holy Ghost; in the sense perhaps that everything that makes a person known may be called his word, and so in that way the Holy Ghost may be called the Son's Word, because He manifests the Son.
Source: Summa Theologica (New Advent)