Greek κενόν (kenon); Latin inane (Lucretius)
Empty space. Void must exist, because without it bodies would have nowhere to be and nothing to move through. Together with atoms it makes up the whole of reality: there is no third kind of thing. See Other Than Atoms and Space There Is Nothing Else.
1. Key Texts
Moreover, the totality of things is bodies and void. That bodies exist is universally witnessed by sensation itself, by reference to which it is necessary to judge by reason what is non-evident, as I have already said above. And if there were not that which we name void and room and intangible substance, bodies would not have anywhere to be or to move through in the way in which they are seen to move. Beyond these nothing can be conceived, either by comprehensive grasp or analogically to things grasped, as fully substantial entities, and not rather spoken of as the accidents or properties of such entities. (tr. Long & Sedley)