Backbiting (ghība) is mentioning another in his absence with what he would dislike, even if it is true. And if it is not true, then it is slander (buhtān), and that is a greater sin.1
The Messenger of Allah ﷺ said:
Whoever defends the honour of his brother when he is being backbitten, Allah makes it a right upon Himself to free him from the fire.
Jāmi as-Saghīr, Imam as-Suyūti
1] Al-Hadiyah al-Alā’iyyah, Alāudīn ibn Muhammad Ameen Ābideen, pg. 332 (Dar al-Faiha)
Orthodox Muslim 