Emptiness is a concept which describes both the ultimate view of the Buddha's mind and the relative view of impermance. It is not a void or nebulous nothing.

Emptiness is a loose translation of the Sanskrit and Tibetan words that are used to define the nature of enlightenment.  The nature of enlightenment is empty or devoid of extremes of eternalism and nihilism.  The nature of enlightenment is devoid of aversion and attachment, and it is beyond words and concepts.  The nature of enlightenment recognizes the impermanence of existence. The nature of enlightenment knows that the selfish notion of an eternal ego is like a fanatical cult.  The nature of enlightenment understands that the perception of a single individual does not represent the totality of existence.

This is what emptiness is:  A true perception.  One truly sees how everything is. This this is the special clarity which arises with emptiness.