Objection 1. It would seem that derision is not a special sin distinct from those mentioned above. For laughing to scorn is apparently the same as derision. But laughing to scorn pertains to reviling. Therefore derision would seem not to differ from reviling.
Objection 2. Further, no man is derided except for something reprehensible which puts him to shame. Now such are sins; and if they be imputed to a person publicly, it is a case of reviling, if privately, it amounts to backbiting or tale-bearing. Therefore derision is not distinct from the foregoing vices.
Objection 3. Further, sins of this kind are distinguished by the injury they inflict on one's neighbor. Now the injury inflicted on a man by derision affects either his honor, or his good name, or is detrimental to his friendship. Therefore derision is not a sin distinct from the foregoing.
On the contrary, Derision is done in jest, wherefore it is described as "making fun." Now all the foregoing are done seriously and not in jest. Therefore derision differs from all of them.
I answer that, As stated above (Question 72, Article 2), sins of word should be weighed chiefly by the intention of the speaker, wherefore these sins are differentiated according to the various intentions of those who speak against another. Now just as the railer intends to injure the honor of the person he rails, the backbiter to depreciate a good name, and the tale-bearer to destroy friendship, so too the derider intends to shame the person he derides. And since this end is distinct from the others, it follows that the sin of derision is distinct from the foregoing sins.
Reply to Objection 1. Laughing to scorn and derision agree as to the end but differ in mode, because derision is done with the "mouth," i.e. by words and laughter, while laughing to scorn is done by wrinkling the nose, as a gloss says on Psalm 2:4, "He that dwelleth in heaven shall laugh at them": and such a distinction does not differentiate the species. Yet they both differ from reviling, as being shamed differs from being dishonored: for to be ashamed is "to fear dishonor," as Damascene states (De Fide Orth. ii, 15).
Reply to Objection 2. For doing a virtuous deed a man deserves both respect and a good name in the eyes of others, and in his own eyes the glory of a good conscience, according to 2 Corinthians 1:12, "Our glory is this, the testimony of our conscience." Hence, on the other hand, for doing a reprehensible, i.e. a vicious action, a man forfeits his honor and good name in the eyes of others--and for this purpose the reviler and the backbiter speak of another person--while in his own eyes, he loses the glory of his conscience through being confused and ashamed at reprehensible deeds being imputed to him--and for this purpose the derider speaks ill of him. It is accordingly evident that derision agrees with the foregoing vices as to the matter but differs as to the end.
Reply to Objection 3. A secure and calm conscience is a great good, according to Proverbs 15:15, "A secure mind is like a continual feast." Wherefore he that disturbs another's conscience by confounding him inflicts a special injury on him: hence derision is a special kind of sin.
Source: Summa Theologica (New Advent)