Passions of the soul

The OED defines "affection" as:

I. Senses relating to the mind. 1. a. The action or result of affecting the mind in some way; a mental state brought about by any influence; an emotion, feeling.

By "affection," it seems you mean what St. Thomas calls "passion" (passio) in his Treatise on the Passions (Summa Theologica I-II qq. 22-48). Passio literally means "suffering" (as in Passio Christi, the Passion of Christ), but more generally it means "passive"—i.e., able to suffer something, receive something from an active agent.

Summa Theologica I-II q. 22 a. 1 ("Whether any passion is in the soul?") c.:

The word "passive" [pati = "to be suffered"] is used in three ways. First, in a general way, according as whatever receives something is passive, although nothing is taken from it: thus we may say that the air is passive when it is lit up [because it receives the light]. But this is to be perfected rather than to be passive. Secondly, the word "passive" [pati] is employed in its proper sense, when something is received, while something else is taken away: and this happens in two ways. For sometimes that which is lost is unsuitable to the thing: thus when an animal's body is healed, and loses sickness. At other times the contrary occurs: thus to ail is to be passive; because the ailment is received and health is lost. And here we have passion in its most proper acceptation. For a thing is said to be passive from its being drawn to the agent: and when a thing recedes from what is suitable to it, then especially does it appear to be drawn to something else. Moreover in De Generat. i, 3 it is stated that when a more excellent thing is generated from a less excellent, we have generation simply, and corruption in a particular respect: whereas the reverse is the case, when from a more excellent thing, a less excellent is generated. In these three ways it happens that passions are in the soul. For in the sense of mere reception, we speak of "feeling and understanding as being a kind of passion" (De Anima i, 5). But passion, accompanied by the loss of something, is only in respect of a bodily transmutation; wherefore passion properly so called cannot be in the soul, save accidentally, in so far, to wit, as the [body+soul] "composite" [compositum] is passive. But here again we find a difference; because when this transmutation is for the worse, it has more of the nature of a passion, than when it is for the better: hence sorrow is more properly a passion than joy.

Dogma that a human is a body+soul composites

As I mentioned here, the Council of Vienne (1311-1312), under the authority of Clement V, defined the dogma that the soul is the form of the human body (Denzinger 481):

Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.

"Form" is "the actualizing principle that makes a thing to be what it is." Thus, a body without the substantial form of a human being (a soul) is a corpse.

The Oxford English Dictionary defines form as:

In the Scholastic philosophy: The essential determinant principle of a thing; that which makes anything (matter) a determinate species or kind of being; the essential creative quality.

^{This use of form (Aristotle's μορϕή or εἶδος) and matter (ὕλη) is a metaphorical extension of their popular use. In ordinary speech, a portion of matter, stuff, or material, becomes a ‘thing’ by virtue of having a particular ‘form’ or shape; by altering the form, the matter remaining unchanged, we make a new ‘thing’. This language, primarily applied only to objects of sense, was in philosophical use extended to objects of thought: every ‘thing’ or entity was viewed as consisting of two elements, its form by virtue of which it was different from, and its matter which it had in common with, others.}

Read St. Thomas's short work On the Principles of Nature (De principiis naturæ) for more information on matter and form (hylemorphism).