Adjective.
e.g. "Almighty"
not "Father" but rather "Our Father"
not "King" but rather "King of Kings"
not "Spirit" but rather "Holy Spirit"
not "Knowing" but rather "All Knowing"
God is referred to in the superlative, which a quality or degree.
When it is said "God is good" it is not meant simply that God happens to be a thing that is good or an action that is good. Hence, Mark 10:18:
"Why do you call me good?" Jesus answered. "No one is good--except God alone.
God is not merely 'a' god, He is 'the' God. The word 'the' is used in the superlative sense.