What is God?...

God is a word that means different things to different people. To many Taoist or Buddhist the word is not part of their religion's glossary. To Hindus that word has a different meaning than it does to a Christian. Muslims have a different perspective and so do the Jews. So for the purpose of The Mystic Doctrines website, we need a common definition of God.
Quoting different sources from each religion before revealing the definition.


Taoism
Tao, the subtle reality of the universe cannot be described, That which can be described in words is mearly a conception of the mind. Although names and descriptions have been applied to it, the subtle reality is beyond the description.
Tao Teh Ching - beginning of chapter 1

The subtle essense of the universe is elusive and evasive.
...
It is the subtle origin of the whole of creation and non-creation. It existed prior to the beginning of time as the deep and subtle reality of the universe. It brings all into being.
Tao Teh Ching - portions of chapter 21


Buddhism
"There is, O monks, an unborn, unoriginated, uncreated, unformed. Were there not, O monks, this unborn, unoriginated, uncreated, unformed, there would be no escape from the world of the born, originated, created, formed.

"Since, O monks, there is an unborn, unoriginated, uncreated, and unformed, therefore is there an escape from the born, originated, created, formed."
The Gospel of Buddha - Sermon at the bamboo grove at Rajagaha


Hinduism
Neither the multitude of gods nor great sages know of my origin, for I am the source of all the gods and great sages.

A mortal who knows me as the unborn, beginningless great lord of the worlds is freed from all delusion and all evils.
The Bhagavad-Gita - The tenth teaching, verses 2 & 3

Sihkism
There is One, only One Supreme Being, Truth Eternal, Creator of all seen & unseen, Fearless, Without hatred, Timeless Being, Non-Incarnated, Self created, Realized by the Grace of Guru (Perfect Master Only.)
Guru Granth Sahib Page 1

Judaism
In the beginning God created the heavens and the earth.
Genesis 1:1

For thus saith the Eternal that created the heavens; God himself that formed the earth and made it; he hath established it, he created it not in vain, he formed it to be inhabited: I am the self existent One; and there is none else.
Isaiah 45:18


Christianity
In the beginning was the Word, and the Word was with God, and the Word was God Himself. He was present originally with God. All things were made and came into existence through Him; and without him was not one thing made that has come into being.
Gospel of John 1:1-3


Sufism
You are the Absolute Existence which causes (our) transient (existences) to appear.
Masnavi - Book 1 - Creator and Creation
Now, a definition of God.

God is the indescribable, uncreated, self existent, eternal all knowing source of all reality and being.................from the Reluctant Messenger