He who is single, absolutely without partner or equal in His essence, attributes, actions, names and decrees.
“O People of the Book! commit no excesses in your religion: nor say of Allah aught but the truth. Christ Jesus the son of Mary was (no more than) A Messenger of Allah, and His Word, which He bestowed on Mary, and a Spirit proceeding from Him: so believe in Allah and His Messengers. Say not “Trinity”: desist: it will be better for you: FOR ALLAH IS ONE GOD: GLORY BE TO HIM: (FAR EXALTED IS HE) ABOVE HAVING A
SON. To Him belong all things in the heavens and on earth. And enough is Allah as a Disposer of affairs.”
(4:171)The Holy Quran
From the root w-h-d with the classical Arabic connotations: to be one, to be made one, to be called one to connect, join, unite to be unique, singular to be one apart from others.