al-Ahad

The One who has ever been and ever remains alone. The incomparable, unequalled, indivisible One. The One who is the essence of Unity. The Sole One. The One and Only One.

The One who has no second [that shares] in the lordship, nor in the essence, nor in the attributes. The One who was not begotten and has never begotten.

The One who depends upon no other, and to Whom there is no likeness. The One in whom all names, attributes and relationships are united.

From the root a-h-d which has the following classical Arabic connotations:

to be one
to be the only one, one alone, sole
to unite, unify

This name is used in the Qur'ān. For example, see 112:1

The ancient roots of Ahad point toward to the essential qualities which describe Unity, while the ancient roots of Wāhid point toward the manifestations which arise from that Unity.

That which is Ahad cannot be divided into parts, or any parts distinguished. That which is Wāhid has the appearance of being divisible into separate parts, or being comprised of separate parts.

Ahad is used to refer to the One, the sole One, the One who was not begotten and who has ever been alone, the One who has no second, the Indivisible.

Wāhid indicates the solitary Starting Point, the One Source, the First of the Many, from which all of creation has arisen as manifestations of Unity. From a human point of view, the manifestations may appear separate and diverse, but in reality they are One.