As A.W.Pink so rightly says "Here we enter a realm which transcends the finite mind, and where speculation is profane". Because God is infinite, and the human mind is finite (as well a fallen) we cannot possibly adequately define God. We can however give a definition of God insofar as we know Him, and what we know about Him from scripture. I have not been able to find a better summary of the doctrine of God than that taught in the Westminster Shorter Catechism which I will quote here: " God is spirit, infinite, eternal, and unchangeable, in His being, wisdom, power, holiness, justice, goodness, and truth". Strong's definition is also very good; "God is the infinite and perfect Spirit in whom all things have their source, support, and end".

God is the Creator and Sustainer of  all things, both visible and invisible.  God is eternally existent in three persons; Father, Son, and Holy Spirit, co-equal in power and glory.

I am God, and there is none else; I am God, and there is none like me, declaring the end from the beginning, and from ancient times the things that are not yet done, saying, "My counsel shall stand, and I will do all my pleasure". Isaiah 46:9-10