This word has two meanings that should never be forgotten. Most individuals use the word in common speech to refer to time without beginning or everlasting or endless time. In this meaning of the word, God cannot be said to be eternal, for God is not in time and so, is not subject to change or mutability that characterizes all things in time.
The second meaning of eternal is to have being outside of time. Eternal being is beyond our capacity to imagine or conceive in terms that are positive. Our only sense of the eternal in this meaning of the term is negative -- the negation of time. Even if we think we can imagine God, we cannot imagine His eternity.
The eternal is like the immaterial in that it can be grasped by us only in negative terms -- what it is not. Most individuals use the word "spiritual" as a synonym for "immaterial" but they forget that the only meaning they can attach to that word is negative, not positive. The spiritual is simply that which is not material. So, too, the eternal is simply that which is not in time. The eternal is the immutable.
Adler's Philosophical Dictionary (1995)
Great Ideas from the Great Books (1963)
by Mortimer J. Adler