Objection 1. It would seem that matrimony is unfittingly defined in the text* (Sent. iv, D, 27). [The definition alluded to is as follows: "Marriage is the marital union of man and woman involving living together in undivided partnership."] For it is necessary to mention matrimony in defining a husband, since it is the husband who is joined to the woman in matrimony. Now "marital union" is put in the definition of matrimony. Therefore in these definitions there would seem to be a vicious circle.
Objection 2. Further, matrimony makes the woman the man's wife no less than it makes the man the woman's husband. Therefore it should not be described as a "marital union" rather than an uxorial union.
Objection 3. Further, habit [consuetudo] pertains to morals. Yet it often happens that married persons differ very much in habit. Therefore the words "involving their living together [consuetudinem] in undivided partnership" should have no place in the definition of matrimony.
Objection 4. Further, we find other definitions given of matrimony, for according to Hugh (Sum. Sent. vii, 6), "matrimony is the lawful consent of two apt persons to be joined together." Also, according to some, "matrimony is the fellowship of a common life and a community regulated by Divine and human law"; and we ask how these definitions differ.
I answer that, As stated above (Article 2), three things are to be considered in matrimony, namely its cause, its essence, and its effect; and accordingly we find three definitions given of matrimony. For the definition of Hugh indicates the cause, namely the consent, and this definition is self-evident. The definition given in the text indicates the essence of matrimony, namely the "union," and adds determinate subjects by the words "between lawful persons." It also points to the difference of the contracting parties in reference to the species, by the word "marital," for since matrimony is a joining together for the purpose of some one thing, this joining together is specified by the purpose to which it is directed, and this is what pertains to the husband [maritum]. It also indicates the force of this joining--for it is indissoluble--by the words "involving," etc.
The remaining definition indicates the effect to which matrimony is directed, namely the common life in family matters. And since every community is regulated by some law, the code according to which this community is directed, namely Divine and human law, finds a place in this definition. while other communities, such as those of traders or soldiers, are established by human law alone.
Reply to Objection 1. Sometimes the prior things from which a definition ought to be given are not known to us, and consequently certain things are defined from things that are posterior simply, but prior to us; thus in the definition of quality the Philosopher employs the word "such" [quale] when he says (Cap. De Qualitate) that "quality is that whereby we are said to be such." Thus, too, in defining matrimony we say that it is a "marital union," by which we mean that matrimony is a union for the purpose of those things required by the marital office, all of which could not be expressed in one word.
Reply to Objection 2. As stated (2), this difference indicates the end of the union. And since, according to the Apostle (1 Corinthians 11:9), the "man is not [Vulgate: 'was not created'] for the woman, but the woman for the man," it follows that this difference should be indicated in reference to the man rather than the woman.
Reply to Objection 3. Just as the civic life denotes not the individual act of this or that one, but the things that concern the common action of the citizens, so the conjugal life is nothing else than a particular kind of companionship pertaining to that common action. wherefore as regards this same life the partnership of married persons is always indivisible, although it is divisible as regards the act belonging to each party.
The Reply to the Fourth Objection is clear from what has been said above.
Source: Summa Theologica (New Advent)