What is backbitting?

Islamic-laws according to Sayyed Sistani:

Question: What is the proper definition of 'backbiting'? Please, give some references from the Quran and Hadith also.

Answer: Backbiting means “speaking ill of a believer in their absence with the purpose of disparaging or not, and no matter whether the alleged shortcoming was related to his body, lineage, behaviour, deeds, statements, religion, or life, and other defects which are [usually] concealed from the people. Similarly, it does not matter whether the description was done by words or by gesture.”

Almighty Allãh condemned backbiting in His noble Book and has described it such that mind and body feel abhorrence towards it. He said, “And some of you should not backbite the others: would anyone of you like to eat the flesh of his dead brother? No, you abhor it.” (Al-Quran 49:12)

The Prophet (s.a.w.) said, “Be careful of backbiting because backbiting is worse than adultery, in that a person who commits adultery can repent and ask forgiveness from God, and Allãh can forgive him whereas Allãh will not forgive the backbiter until the person who was at the receiving end forgives him.”

It is not appropriate for a believer to listen to backbiting against his believing brother. Indeed, it appears from the sayings of the Prophet and the Imams (may Allãh bless them all) that it is obligatory upon one who hears backbiting to support the person who is being disparaged; and that if he does not repel the backbiting [against his believing brother], Allãh will abandon him in this world as well as in the hereafter, and he shall be held accountable just like the one who did the backbiting.