Nothing. The absence of anything, any action, description, idea, or even a definition. Nothing is not a thing. Nothing is not describable. Nothing is not comparable to anything. Nothing is in the name. "No thing". There is no thing in nothing, in both word and definition.
Many describe "nothing" as a black void. This is not true. A void would be a thing, so nothing would definitely have a thing. Nothing is not a black void, because that would no longer classify it as "nothing". A void is a thing. Nothing isn't.
"Nothing" cannot have a size.
Nothing cannot have a value or definition.
Nothing cannot have a concept.
Nothing cannot have a proof of existence.
Nothing must be absent of all adjectives, actions, ideas.
Nothing must be nowhere, never, impossible.
Nothing cannot have mass, density, or temperature.
Nothing cannot be a thing or contain a thing.
Nothing must not have any properties of above.
Nothing would be beyond all these descriptions which don't fully define the true meaning of Nothing.
If nothing is surrounded by things, a thing can pass into the nothing and thus would make the nothingness have a thing making there no longer be a nothing there. It would be a thing. This is why nothings are impossible in our universe.
A nothing is impossible from our current understandings of the universe due to quantum fluctuations and virtual particles popping in and out of existence.
Any matter / anti-matter inside of a nothing would make that once-nothing a thing so it would stop being a thing because it contained something and violates what a nothing can be.
If nothing has any stats that are existent then it is not a nothing because a nothing is a nothing and not a thing and no things can be in nothing.