He is the one who is singled out in all aspects of perfection such that nothing else shares with Him in these. It is obligatory upon the servants to single Him out alone in belief, saying and action by acknowledging His unrestricted perfection, His uniqueness and singling Him out alone for all types of worship.
“'O two companions of the prison! Are many different lords better or Allah, al-Wahid, the Irresistible?'”
[Yusuf (12): 39]
“Say: He is Allah, al-Ahad.”
[al-Ikhlas (112): 1]
From “Explanation to the Beautiful and Perfect Names of Allah” — Shaykh as-Sa’di (d.1376H)