What do we really mean by "God"??
That depends on who "we" refers to, I guess. Different people often intend the word to mean very different things. Even individual people will often seem to use the word inconsistently, intending it to mean different things at different times, in different rhetorical circumstances.
People will often use the word "God" to refer to the central supernatural characters in religious myths. When the word "god" is capitalized as "God", it's usually expected to imply a monotheistic deity, which is imagined as being the only one of its supernatural kind. There's usually some unstated assumption at that point that monotheism is better than polytheism, but there's rarely any attempt to justify it.
People who use the word may or may not privilege a particular tradition or text (the Bible, the Quran, the Gita) by assuming that it uniquely reveals this singular supernatural character and describes the details "his" (these mythological characters are almost always imagined as if they were a "person") interactions with particular chosen human beings. People will often just assume that "God" possesses purposes, desires and commands, and that "he" has revealed these to humans. People will assume that particular books, traditions and churches know about these things and exist to further the divine purposes. But while the general pattern is fairly consistent, the actual details are often all over the map, depending on who we are asking. (A Roman Catholic layperson in Nicaragua, a Shi'ite cleric in Qom, a Biblical Baptist in rural Mississippi, a Vishnaivite devotee in Southern India.)
I think that even atheists will typically, and often unconsciously be assuming lots of this stuff, whenever they use the word "God". What atheists intend the word to mean often depends on their own individual circumstances. Atheists are often reacting against the traditions of their families and of their surrounding cultures, and not every atheist comes from the same place.
Why do we use a word like God for things that are clearly unlike a God? Some people believe God is an impersonal energy or omnipresent supernatural force. Others conceive of a being beyond spacetime in some higher dimensional mode of existence. Aren't we stretching the term God here beyond the boundaries of its original meaning and scope?
And then there's the so-called "God of the philosophers". This is the God transformed from a blustering super-person thundering on a Middle Eastern mountaintop, into a set of abstract quasi-technical functions and qualities. God is imagined as embodying perfections, as being omniscience, omnipotence, supreme goodness, as being that which is eternal (as opposed to transitory) and necessary (as opposed to contingent). God becomes the universe's first-cause and its continued sustainer. This way of thinking ultimately derives from the ideas of ancient Greek philosophers like Plato and Aristotle, and from the Christian (and Muslim) philosophical theologians that adoped the philosophical way of thinking in late antiquity and early medieval times. It isn't always clear what relationship this "God" has to the characters of Biblical (or Vedic or Quranic) mythology, or even whether it's always consistent with that much older and far more literary mode of imagination.
I think that when people in the Western world use the English word "God", they are usually kind of implicitly assuming all the diverse and not-always-coherent cultural baggage that's become attached to the word over the last 2,000 years of mostly Christian history. People, even professional theologians and philosophers of religion, will veer back and forth between the various ways of thinking about the meaning of the word "God", assuming without much attempt at argument or justification that their philosophical arguments about first-causes, necessary-existents, supreme perfections or apparent design, are somehow connected-with and relevant-to the Bible and to Jesus Christ's saving incarnation. People in other cultures will assume just as easily that very similar philosophical arguments are somehow connected with their own culture's Quran, Allah, Vishnu and/or Krishna.
I don't believe that there's any hidden connection tying all of it together, any underlying logical framework of mutual-implication there for scholars to discover, that ties all of the many usages included in the meaning of the word "God" into one single consistent and coherent whole.
Traditions on the scale of religions more closely resemble thought-tsunamis, waves of fragmentary and often unconnected intellectual debris washing down through time.