From “Sanctifying Grace” on NewAdvent:
Moreover, sanctifying grace as an active reality, and not a merely external relation, must be philosophically either substance or accident. Now, it is certainty not a substance which exists by itself, or apart from the soul, therefore it is a physical accident inhering in the soul, so that the soul becomes the subject in which grace inheres; but such an accident is in metaphysics called quality (qualitas, poiotes) therefore sanctifying grace may be philosophically termed a “permanent, supernatural quality of the soul”, or, as the Roman Catechism (P. II, cap. ii, de bap., n. 50) says “divina qualitas in anima inhaerens”.
2. Sanctifying grace cannot be termed a habit (habitus) with the same precision as it is called a quality. Metaphysicians enumerate four kinds of quality:
habit and disposition;
power and want of power;
passion and passible quality, for example, to blush, pale with wrath;
form and figure (cf. Aristotle, Categ. VI).
Manifestly sanctifying grace must be placed in the first of these four classes, namely habit or disposition; but as dispositions are fleeting things, and habit has a permanency theologians agree that sanctifying grace is undoubtedly a habit, hence the name: Habitual Grace (gratia habitualis). Habitus is subdivided into habitus entitativus and habitus operativus. A habitus entitativus is a quality or condition added to a substance by which condition or quality the substance is found permanently good or bad, for instance: sickness or health, beauty, deformity, etc. Habitus operativus is a disposition to produce certain operations or acts, for instance, moderation or extravagance; this habitus is called either virtue or vice just as the soul is inclined thereby to a moral good or to a moral evil. Now, since sanctifying grace does not of itself impart any such readiness, celerity, or facility in action, we must consider it primarily as a habitus entitativus, not as a habitus operativus. Therefore, since the popular concept of habitus, which usually designates a readiness, does not accurately express the idea of sanctifying grace, another term is employed, i.e. a quality after the manner of a habit (qualitas per modum habitus), and this term is applied with Bellarmine (De grat. et lib. arbit., I, iii). Grace, however, preserves an inner relation to a supernatural activity, because it does not impart to the soul the act but rather the disposition to perform supernatural and meritorious acts therefore grace is remotely and mediately a disposition to act (habitus remote operativus). On account of this and other metaphysical subtleties the Council of Trent has refrained from applying the term habitus to sanctifying grace.