Emptiness is a state of quietude of the mental or vital or all the consciousness not visited by any mind or vital movements, but open to the Pure Existence and ready or tending to be that or already that but not yet realised in its full power of being. Which of these conditions it happens to be depends on the particular case. The Self state or the state of pure Existence is sometimes also called emptiness, but only in the sense that it is a state of sheer static rest of being without any contacts of mobile Nature.
Ref: Letters on Yoga Vol. III