Objection 1. It would seem that transgression is not a special sin. For no species is included in the definition of its genus. Now transgression is included in the definition of sin; because Ambrose says (De Parad. viii) that sin is "a transgression of the Divine law." Therefore transgression is not a species of sin.
Objection 2. Further, no species is more comprehensive than its genus. But transgression is more comprehensive than sin, because sin is a "word, deed or desire against the law of God," according to Augustine (Contra Faust. xxii, 27), while transgression is also against nature, or custom. Therefore transgression is not a species of sin.
Objection 3. Further, no species contains all the parts into which its genus is divided. Now the sin of transgression extends to all the capital vices, as well as to sins of thought, word and deed. Therefore transgression is not a special sin.
On the contrary, It is opposed to a special virtue, namely justice.
I answer that, The term transgression is derived from bodily movement and applied to moral actions. Now a person is said to transgress in bodily movement, when he steps [graditur] beyond [trans] a fixed boundary--and it is a negative precept that fixes the boundary that man must not exceed in his moral actions. Wherefore to transgress, properly speaking, is to act against a negative precept.
Now materially considered this may be common to all the species of sin, because man transgresses a Divine precept by any species of mortal sin.
But if we consider it formally, namely under its special aspect of an act against a negative precept, it is a special sin in two ways. First, in so far as it is opposed to those kinds of sin that are opposed to the other virtues: for just as it belongs properly to legal justice to consider a precept as binding, so it belongs properly to a transgression to consider a precept as an object of contempt. Secondly, in so far as it is distinct from omission which is opposed to an affirmative precept.
Reply to Objection 1. Even as legal justice is "all virtue" (58, 5) as regards its subject and matter, so legal injustice is materially "all sin." It is in this way that Ambrose defined sin, considering it from the point of view of legal injustice.
Reply to Objection 2. The natural inclination concerns the precepts of the natural law. Again, a laudable custom has the force of a precept; since as Augustine says in an epistle On the Fast of the Sabbath (Ep. xxxvi), "a custom of God's people should be looked upon as law." Hence both sin and transgression may be against a laudable custom and against a natural inclination.
Reply to Objection 3. All these species of sin may include transgression, if we consider them not under their proper aspects, but under a special aspect, as stated above. The sin of omission, however, is altogether distinct from the sin of transgression.
Source: Summa Theologica (New Advent)