Obviously, there is no single accepted definition for God.
However there seems to be a common core to how God is viewed in the Hindu Advaita Vedanta, Islamic Sufism, and some Jewish traditions, as reflected by Maimonides, and later in Hasidic Judaism.
That core is roughly that God is immanent, that it is transcendental to human reason and language, that it is impersonal and unchanging, that it is "mirrored" in the human soul, and that most people fail to realize that reality, but have the potential for it.
In Hindu Advaita Vedanta this is asserted directly. Atman is the divine and transcendental nature of human consciousness; Brahman, the Godhead, is the impersonal ultimate nature of reality; and the two are identical.
I can already imagine readers protesting with rage that "this is a philosophy website, not theology!" — but what can you do? Advaita Vedanta is considered Eastern philosophy and the SEP article on Maimonides calls him "the greatest Jewish philosopher of the medieval period" - so swallow your Greek pride and open your mind (pun intended).
The rest of this answer are examples and quotes from each tradition to the traits I have listed above. It is just a small selection of quotes. A full list and discussion could easily grow to the size of a book:
1) That God is immanent:
In Hinduism:
"I am the goal of life, the Lord and support of all, the inner witness, the abode of all." - Bhagavad Gita §9.18
In Judaism:
"Hasidism teaches that while a superficial observance of the universe by the "eyes of the flesh" (Einei ha-Basar) purportedly reflects the reality of all things profane and worldly, a true devotee must transcend this illusory façade and realize that there is nothing but God." - Wikipedia
2) That God is transcendental:
In Hinduism:
"Subtler than the subtlest is this Self, and beyond all logic." - Katha Upanishad
"He truly knows Brahman who knows him as beyond knowledge; he who thinks that he knows, knows not. The ignorant think that Brahman is known, but the wise know him to be beyond knowledge." - Kena Upanishad
In Sufism:
"failure dogs the analytic mind, Which whimpers like a child born deaf and blind." - The Conference of the Birds
"since no words suffice, what use are mine to represent or to describe this sign?" - The Conference of the Birds
In Judaism:
"You must bear in mind, that by affirming anything of God, you are removed from Him in two respects; first, whatever you affirm, is only a perfection in relation to us; secondly, He does not possess anything superadded to this essence; His essence includes all His perfections, as we have shown. Since it is a well-known fact that even that knowledge of God which is accessible to man cannot be attained except by negations, and that negations do not convey a true idea of the being to which they refer, all people, both of past and present generations, declared that God cannot be the object of human comprehension" - Maimonides, Guide for the Perplexed
3) That God is impersonal and unchanging:
In Hiduism:
"Few see through the veil of maya. The world, deluded, does not know that I am without birth and changeless." - Bhagavad Gita §7.25
"My true being is unborn and changeless." - Bhagavad Gita §4.6
"Soundless, formless, intangible, undying, tasteless, odorless, without beginning, without end, eternal, immutable, beyond nature, is the Self." - Katha Upanishad
Note that in Advaita Vedanta Atman (the true transcendental Self) is identical with Brahman.
In Judaism:
"all such [descriptions] and the like which are related in the Torah and the words of the Prophets - all these are metaphors and imagery. [For example,] 'He who sits in the heavens shall laugh' [Psalms 2:4], 'They angered Me with their emptiness' [Deuteronomy 32:21], and 'As God rejoiced' [ibid. 28:63]. With regard to all such statements, our Sages said: 'The Torah speaks in the language of man.' This is [borne out by the rhetorical question (Jeremiah 7:19):] 'Are they enraging Me?' Behold, [Malachi 3:6] states: 'I, God, have not changed.' Now were He to at times be enraged and at times be happy, He would change. Rather, all these matters are found only with regard to the dark and low bodies, those who dwell in houses of clay, whose foundation is dust. In contrast, He, blessed be He, is elevated and exalted above all this." - Maimonides, Yesodei haTorah
4) That God is "mirrored" in the human soul:
In Hinduism:
"In one’s own soul Brahman is realized clearly, as if seen in a mirror." - Katha Upanishad
In Sufism:
"Your heart is not a mirror bright and clear
If there the Simorgh’s form does not appear;
No one can bear His beauty face to face,
And for this reason, of His perfect grace,
He makes a mirror in our hearts – look there
To see Him, search your hearts with anxious care" - The Conference of the Birds
In Judaism:
"And God created man in His image, in the image of God He created him" - Genesis 1
That "in His image" cannot refer to body or psychology follows from Maimonides.
6) That most people fail to realize that reality, but have the potential for it:
In Hinduism:
"The glory of the Self is beheld by a few, and a few describe it; a few listen, but many without understanding." - Bhagavad Gita §1.29
"Few see through the veil of maya. The world, deluded, does not know that I am without birth and changeless.” - Bhagavad Gita §7.25
"The Self is not known through study of the scriptures, nor through subtlety of the intellect, nor through much learning. But by him who longs for him is he known." - Katha Upanishad
In Sufism:
"Wanderer, you are distraught;
Be calm. Our glorious King cannot admit
All comers to His court; it is not fit
That every rascal who sleeps out the night
Should be allowed to glimpse its radiant light.
Most are turned back, and few perceive the throne;
Among a hundred thousand there is one" - The Conference of the Birds
In Judaism:
"And Moses answered and said: ‘But, behold, they will not believe me, nor hearken unto my voice; for they will say: The lord hath not appeared unto thee.’" - Exodus 4
"Hasidism teaches that while a superficial observance of the universe by the "eyes of the flesh" (Einei ha-Basar) purportedly reflects the reality of all things profane and worldly, a true devotee must transcend this illusory façade and realize that there is nothing but God." - Wikipedia
