A good brief intro in the subject, https://www.youtube.com/watch?v=1zMf_8hkCdc, largely following Thomas Aquinas definition. For modern sensibilities it may be hard to stomach that a XIII century monk might have had deeper insights than the reductionist materialist drivel we are soaking in. Alas, I can't help with that.

"God is not a Being in the World, God is Ipsum Esse Subsistens". In English, "God is the subsistent act of 'to be' itself".