Affection is in one sense spoken of a quality in virtue of which a body can be altered, such as white and black, sweet and bitter, heaviness and lightness, and the like.
In another every effect and alteration that already comes about with regard to these, such as being white or black or cold or hot. This so called affection is a certain impermanent change in virtue either of the soul or the body.
Of the former it is the injurious alterations and changes that are called affections to a greater extent — either in virtue of the soul or in virtue of the body — especially those that are concerned with injurious and disagreeable circumstances, such as sickly annoyances and disagreeable annoyances that are mistaken.
Moreover, an extreme sadness or disaster are called affection.
Bibliography: Aristotle, Metaphysics (1022b.15 to 1022b.21), Alexander of Aphrodisias in Metaphysics
Translation, text editing: George Kotsalis