Objection 1. It would seem that clemency and meekness are absolutely the same. For meekness moderates anger, according to the Philosopher (Ethic. iv, 5). Now anger is "desire of vengeance" [Aristotle, Rhet. ii, 2]. Since, then, clemency "is leniency of a superior in inflicting punishment on an inferior," as Seneca states (De Clementia ii, 3), and vengeance is taken by means of punishment, it would seem that clemency and meekness are the same.
Objection 2. Further, Tully says (De Invent. Rhet. ii, 54) that "clemency is a virtue whereby the mind is restrained by kindness when unreasonably provoked to hatred of a person," so that apparently clemency moderates hatred. Now, according to Augustine [Ep. ccxi], hatred is caused by anger; and this is the matter of meekness and clemency. Therefore seemingly clemency and meekness are absolutely the same.
Objection 3. Further, the same vice is not opposed to different virtues. But the same vice, namely cruelty, is opposed to meekness and clemency. Therefore it seems that meekness and clemency are absolutely the same.
On the contrary, According to the aforesaid definition of Seneca (Objection 1) "clemency is leniency of a superior towards an inferior": whereas meekness is not merely of superior to inferior, but of each to everyone. Therefore meekness and clemency are not absolutely the same.
I answer that, As stated in Ethic. ii, 3, a moral virtue is "about passions and actions." Now internal passions are principles of external actions, and are likewise obstacles thereto. Wherefore virtues that moderate passions, to a certain extent, concur towards the same effect as virtues that moderate actions, although they differ specifically. Thus it belongs properly to justice to restrain man from theft, whereunto he is inclined by immoderate love or desire of money, which is restrained by liberality; so that liberality concurs with justice towards the effect, which is abstention from theft. This applies to the case in point; because through the passion of anger a man is provoked to inflict a too severe punishment, while it belongs directly to clemency to mitigate punishment, and this might be prevented by excessive anger.
Consequently meekness, in so far as it restrains the onslaught of anger, concurs with clemency towards the same effect; yet they differ from one another, inasmuch as clemency moderates external punishment, while meekness properly mitigates the passion of anger.
Reply to Objection 1. Meekness regards properly the desire itself of vengeance; whereas clemency regards the punishment itself which is applied externally for the purpose of vengeance.
Reply to Objection 2. Man's affections incline to the moderation of things that are unpleasant to him in themselves. Now it results from one man loving another that he takes no pleasure in the latter's punishment in itself, but only as directed to something else, for instance justice, or the correction of the person punished. Hence love makes one quick to mitigate punishment --and this pertains to clemency--while hatred is an obstacle to such mitigation. For this reason Tully says that "the mind provoked to hatred" that is to punish too severely, "is restrained by clemency," from inflicting too severe a punishment, so that clemency directly moderates not hatred but punishment.
Reply to Objection 3. The vice of anger, which denotes excess in the passion of anger, is properly opposed to meekness, which is directly concerned with the passion of anger; while cruelty denotes excess in punishing. Wherefore Seneca says (De Clementia ii, 4) that "those are called cruel who have reason for punishing, but lack moderation in punishing." Those who delight in a man's punishment for its own sake may be called savage or brutal, as though lacking the human feeling that leads one man to love another.
Source: Summa Theologica (New Advent)