A tribe located to the south of Arabia, to which the prophet Hud is said to have been sent. See Qur’an, vii., 63:
“And to ‘Ad we sent our brother Hud, ‘O my people’ said he, ‘worship God: ye have no other god than Him': Will ye not then fear him?”
“Said the unbelieving chiefs among his people; We certainly perceive that thou art unsound of mind; and we surely deem thee an impostor.’
“He said, ‘O my people! It is not unsoundness of mind in me, but I am an Apostle from the Lord of the Worlds.
“The messages of my Lord do I announce to you, and I am your faithful counsellor.
Marvel ye that a warning hath come to you from your Lord through one of yourselves that He may warn you? Remember how he hath made you the successors of the people of Noah, and increased you in tallness of stature. Remember then the favours, of God that it may haply be well with you.”
“They said, ‘Art thou come to us in order that we may worship one God alone, and leave what our fathers worshipped? Then bring that upon us with which thou threatenest us, if thou be a man of truth.”
“He said, ‘Vengeance and wrath shall suddenly light on you from your Lord. Do ye dispute with We about names that you and your fathers have given your idols, and for which, God hath sent you down no warranty? Wait ye then, and I too will wait with you.”
“And we delivered him, and those who were ,on his side by our mercy, and we cut off, to the last man, those who had treated our signs as lies, and who wore not believers.”
Also, Surah lxxxix., 5 : “Hast thou not seen how thy Lord dealt with ‘Ad at Iram. adorned with pillars, whose like have not been reared in these lands” [HUD, IRAM.]
Based on Hughes, Dictionary of Islam