The term "Word" is properly understood in the Divinity as a Personal name, not as an Essential one, and signifies Intellectual Conception.
The nature of this conception requires that it should proceed from something else, namely, from the knowledge of the one who conceives: therefore, in the Divinity, Word properly signifies somewhat proceeding from Another. As such it belongs to the order of Personal names, or those which distinguish the Divine Persons according to Origin; and since the Word proceeds by intellectual emanation, it is the proper name of the Son, and His Person. And since there is no accident in God, whatever exists in the Divine Nature is subsistent: hence the Son only is called the Word, and such Procession is termed Generation.
But in the name Word reference is contained also to creatures, for in knowing Himself God knows all things; and as by one act He knows Himself and everything else, so by His One Word He has expressed, not the Father only, but also creatures. And as God's knowledge of Himself is intellectual only, while His knowledge of creatures is both intellectual and operative, so with the Word of God; as regards the Father It is expressive only, but in regard of creatures It is both expressive and operative. Hence we read in the Psalms: "He spoke and it was done," because the operative conception of the things which God does is contained in the Word.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni