Wahdat al-Wujud is the Arabic term used by Sufi philosophers and theologians to express the concept of God's "Uniqueness of Being." This concept holds that nothing exists in a way that is independent and enduring except for the Ultimate Reality, which is God. The universe and human existence depend on God, but God does not depend is on them; they can only exist because of His creative power. The existence of God is the only Real (in the Sufi sense of Real/permanent/independent versus the earthly or Unreal/temporary/dependent/contingent on God) Truth (haqq in Arabic).
In spite of the fact that he never used the term wahdat al-wujud, the medieval philosopher Ibn Arabi (d. 1240) is often cited as the first to articulate the doctrine associated with it. According to Ibn Arabi, Being/wujud is the basis for all existence, and God alone has True Being. His existence is uniquely unlimited, while everything else in the universe is subject to limitation and constraint. Where God is infinite, everything else is finite.
Sufi philosophy can be esoteric and often appear paradoxical. For example, it is central to the principle of God's unlimited Being that if He wished, He could become limited or constrained, because to hold otherwise would be to place limits on God's power. This is reminiscent of the old question, "If God is all powerful, could he create a stone that He could not lift?" This could be seen as a potential weakness in the argument for God's Unity, in the potential for a question such as, "Could God make Himself multiple, if He wanted to?" Sufis answer this by saying that it is a nonsensical question, because God, being unique in His unlimitation, cannot be described simply as unlimited or limited. In other words, the standards of normal delimitation do not apply to God.
Alongside God's Unity of Being is the Sufi concept of Necessary Being, wajib al-wujud, which means that God's existence, which is the only independent, unlimited Being in itself, is the basis for all other existence. In other words, the world exists because of God, but it does not have to exist. According to this belief, the only Being who has to exist is God. On the highest level, Being is the Absolute Reality of God, the Essence of the Real. The cosmos or the universe as we know it, comprises everything that is not God.
It is important to remember that the God referred to by Sufis is not different from Allah, or the God referred to in broader Islamic cosmological or theological frameworks. It is the same Divine being, but Sufis interpret the meaning of God in their own unique way. For example, all Muslims subscribe to the belief that God has ninety-nine attributes, called his Divine Names, which range from "The All-Knowing" to "the All Powerful" to "The Most Merciful." Sufis interpret these attributes, as do all Muslims, with respect to their own particular practice. For instance, a Sufi interpretation of God's omnipresence and omniscience lends itself to the Sufi maxim that one should "worship God as if one sees Him even though one does not." This is not something that would seem out of the ordinary to a non-Sufi Muslim, however it finds particular expression within the Sufi tradition.