Objection 1. It would seem that love is the same as dilection. For Dionysius says (Div. Nom. iv) that love is to dilection, "as four is to twice two, and as a rectilinear figure is to one composed of straight lines." But these have the same meaning. Therefore love and dilection denote the same thing.
Objection 2. Further, the movements of the appetite differ by reason of their objects. But the objects of dilection and love are the same. Therefore these are the same.
Objection 3. Further, if dilection and love differ, it seems that it is chiefly in the fact that "dilection refers to good things, love to evil things, as some have maintained," according to Augustine (De Civ. Dei xiv, 7). But they do not differ thus; because as Augustine says (De Civ. Dei xiv, 7) the holy Scripture uses both words in reference to either good or bad things. Therefore love and dilection do not differ: thus indeed Augustine concludes (De Civ. Dei xiv, 7) that "it is not one thing to speak of love, and another to speak of dilection."
On the contrary, Dionysius says (Div. Nom. iv) that "some holy men have held that love means something more Godlike than dilection does."
I answer that, We find four words referring in a way, to the same thing: viz. love, dilection, charity and friendship. They differ, however, in this, that "friendship," according to the Philosopher (Ethic. viii, 5), "is like a habit," whereas "love" and "dilection" are expressed by way of act or passion; and "charity" can be taken either way.
Moreover these three express act in different ways. For love has a wider signification than the others, since every dilection or charity is love, but not vice versa. Because dilection implies, in addition to love, a choice [electionem] made beforehand, as the very word denotes: and therefore dilection is not in the concupiscible power, but only in the will, and only in the rational nature. Charity denotes, in addition to love, a certain perfection of love, in so far as that which is loved is held to be of great price, as the word itself implies [Referring to the Latin "carus" (dear)].
Reply to Objection 1. Dionysius is speaking of love and dilection, in so far as they are in the intellectual appetite; for thus love is the same as dilection.
Reply to Objection 2. The object of love is more general than the object of dilection: because love extends to more than dilection does, as stated above.
Reply to Objection 3. Love and dilection differ, not in respect of good and evil, but as stated. Yet in the intellectual faculty love is the same as dilection. And it is in this sense that Augustine speaks of love in the passage quoted: hence a little further on he adds that "a right will is well-directed love, and a wrong will is ill-directed love." However, the fact that love, which is concupiscible passion, inclines many to evil, is the reason why some assigned the difference spoken of.
Reply to Objection 4. The reason why some held that, even when applied to the will itself, the word "love" signifies something more Godlike than "dilection," was because love denotes a passion, especially in so far as it is in the sensitive appetite; whereas dilection presupposes the judgment of reason. But it is possible for man to tend to God by love, being as it were passively drawn by Him, more than he can possibly be drawn thereto by his reason, which pertains to the nature of dilection, as stated above. And consequently love is more Godlike than dilection.
Source: Summa Theologica (New Advent)