- O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin. And do not spy or backbite each other. Would one of you like to eat the flesh of his brother when dead? You would detest it. And fear Allah ; indeed, Allah is Accepting of repentance and Merciful. (Qurʾān 49:12)
Ghībah is the sin of speaking maliciously of someone absent, regardless if the information is true or not. It can be translated as backbiting, gossip, scandal, tattling.
The prophet Muḥammad ﷺ said: “Ghībah involves talking about your brother in a way he doesn’t like.” Someone asked, ‘What if is your opinion about this that if I actually find (that failing) in my brother which I made a mention of?’ He said: ‘If (that failing) is actually found (in him) what you assert, you in fact did Ghībah to him. If that is not in him it is Buhtān.” (reported by Abū Hurayrah in Ṣahḥīḥ Muslim) http://sunnah.quran.com/view/urn/262650
A man once asked the Prophet to define ghībah for him. The Prophet replied, “It's mentioning your brother in a way that he doesn’t like.” The man then asked, “What if what I said about my brother was true?” The Prophet answered, “If what you said about him was true, then you would still have engaged in backbiting, and if what you said about him was false, then you would have slandered him.” (Abu Dawud, Tirmidhi)
If you have committed Ghībah against somebody, do tawbah and seek their forgiveness.
Muḥammad, the Messenger of Allāh ﷺ said, “The person who eats the flesh (i.e. makes ghîbah) of his fellow Muslim in this world, then on the day of judgement, Allāh will present the flesh of a dead person and tell him, ‘Just as you had eaten the flesh of a living person, eat that of a dead person as well. This person will eat it with a frown and continue yelling."
L-M-Z: to criticize, to find fault, to insult; to defame, reproach, blame, find fault, speak ill, slanderer, strike, repel, traduce.