Objection 1. It would seem that backbiting is not as defined by some [Albert the Great, Sum. Theol. II, cxvii.], "the blackening of another's good name by words uttered in secret." For "secretly" and "openly" are circumstances that do not constitute the species of a sin, because it is accidental to a sin that it be known by many or by few. Now that which does not constitute the species of a sin, does not belong to its essence, and should not be included in its definition. Therefore it does not belong to the essence of backbiting that it should be done by secret words.
Objection 2. Further, the notion of a good name implies something known to the public. If, therefore, a person's good name is blackened by backbiting, this cannot be done by secret words, but by words uttered openly.
Objection 3. Further, to detract is to subtract, or to diminish something already existing. But sometimes a man's good name is blackened, even without subtracting from the truth: for instance, when one reveals the crimes which a man has in truth committed. Therefore not every blackening of a good name is backbiting.
On the contrary, It is written (Ecclesiastes 10:11): "If a serpent bite in silence, he is nothing better that backbiteth."
I answer that, Just as one man injures another by deed in two ways--openly, as by robbery or by doing him any kind of violence--and secretly, as by theft, or by a crafty blow, so again one man injures another by words in two ways--in one way, openly, and this is done by reviling him, as stated above (Question 72, Article 1)--and in another way secretly, and this is done by backbiting. Now from the fact that one man openly utters words against another man, he would appear to think little of him, so that for this reason he dishonors him, so that reviling is detrimental to the honor of the person reviled. On the other hand, he that speaks against another secretly, seems to respect rather than slight him, so that he injures directly, not his honor but his good name, in so far as by uttering such words secretly, he, for his own part, causes his hearers to have a bad opinion of the person against whom he speaks. For the backbiter apparently intends and aims at being believed. It is therefore evident that backbiting differs from reviling in two points: first, in the way in which the words are uttered, the reviler speaking openly against someone, and the backbiter secretly; secondly, as to the end in view, i.e. as regards the injury inflicted, the reviler injuring a man's honor, the backbiter injuring his good name.
Reply to Objection 1. In involuntary commutations, to which are reduced all injuries inflicted on our neighbor, whether by word or by deed, the kind of sin is differentiated by the circumstances "secretly" and "openly," because involuntariness itself is diversified by violence and by ignorance, as stated above (65, 4; I-II, 6, 5,8).
Reply to Objection 2. The words of a backbiter are said to be secret, not altogether, but in relation to the person of whom they are said, because they are uttered in his absence and without his knowledge. On the other hand, the reviler speaks against a man to his face. Wherefore if a man speaks ill of another in the presence of several, it is a case of backbiting if he be absent, but of reviling if he alone be present: although if a man speak ill of an absent person to one man alone, he destroys his good name not altogether but partly.
Reply to Objection 3. A man is said to backbite [detrehere] another, not because he detracts from the truth, but because he lessens his good name. This is done sometimes directly, sometimes indirectly. Directly, in four ways: first, by saying that which is false about him; secondly, by stating his sin to be greater than it is; thirdly, by revealing something unknown about him; fourthly, by ascribing his good deeds to a bad intention. Indirectly, this is done either by gainsaying his good, or by maliciously concealing it, or by diminishing it.
Source: Summa Theologica (New Advent)