Dhun-Noorayn (translit. Dhu’n-Nurayn, Dhun-Nurain) means The One with Two Lights. It was a nickname given to Uthman ibn Affan with a few reasons suggested for the name.
Due To Marriage
Uthman was married to two daughters of Prophet Muhammad (ﷺ). First to Ruqayyah, and then upon her death, he married Umm Kulthoom.
Badr al-Deen al-‘Ayni related in his commentary on Saheeh al-Bukhari that Al-Muhallab ibn Abu Sufrah was asked,
Why was Uthman called Dhu’n-Noorayn?. He said,
Because we did not know anyone who married two daughters of a Prophet except him. ‘Umdat al-Qari Sharh Saheeh al-Bukhari, 16/201As-Sallabi, op. cit., pg.12
Abd-Allah ibn Umar ibn Aban al-Ju’fi said,
My maternal uncle, Husyan Al-Ju’fi said to me, ‘O my son, do you know why Uthman was called Dhu’n-Noorayn? I said, ‘I do not know.’ He said, ‘No one else ever married to two daughters of a Prophet, from the time of Allah created Adam until the Hour will begin, except Uthman. Hence he was called Dhun-Noorayn. Sunan al-Bayhaqi, 7/73. Dr. ‘Aatif Lamadah said: a hasan reportAs-Sallabi, op. cit., pg.12
Due to Habits
Another reason given for the nickname Dhu’n-Noorayn is because he would recite a great deal of the Quran every night, and the Quran is light and praying Qiyam al-Layl is light. ‘Uthman ibn ‘Affan Dhu’l-Noorayn, by ‘Abbas al-‘Aqqad, p. 79As-Sallabi, op. cit., pg.12