To define eternity in words is like trying to explain sunshine by shining a UV lamp in someone’s face; it just doesn’t quite have the full effect.
Maybe the closest definition I can give you as a finite being is that eternity is something of a continuous and everlasting motion, that the true nature of time and space are so intimately unknown to us that all we can do is build concepts with words, each time creating a finite model of an infinite system. The only way eternity can be understood is to become so at one with the present moment that the parameters of space and time as we know it break away, leaving us bare in full sight of the all-pervasive reality within and throughout all that is and is not. Eternity is oneness, void, nothing and all.
Only when we find the eternal within ourselves can we understand this.
“Eternity is the now that never passes away” Augustine