If you are refering to space it would mean a defined area with no atoms, electrons, protons... parts of particles, energies (heat energy (if an atom was inside of nothing the electrons would not orbit around it, they would just be stuck, -400K), light (which is a particle traveling at the speed of light), any "wave" of energy(even though we dont know them all yet, like gravity which is created by gravitons in particles (one particle pulls on another no matter how far away, eventually they will join).
Chances are the only "Nothingness/Nothing" is on the outside of our universe as it expands outwards at the speed of light. (if you believe in the big bang) There is no way to pull a vacume that strong, keep it that cold, and not allow any "energies" to pass through, all you would end up with is an almost empty defined area.
Assuming the formula for Gravity is correct, the universe will continue expanding untill all the molecules and energies are pulled back in by their own gravity.
This "theory" of nothing, could all be easier to explain by teaching the big bang theory, then tieing two balls together with a string of rubber, have two people throw the two balls in oppisite directions, they will fly back to where they were thrown from.
Just past the maximum distance one of the balls flew from where they were thrown would be Nothing, or Nothingness, so to define nothing:
Nothing or Nothingness would be the area outside the maximum distance (radius from origin of big bang) any molecule or energy will ever reach before stopping and being pulled back in towards the center of the universe due to gravity. (people have theorized that when this shift happens time will reverse and rewind, thus leaving us to wonder has the universe been repeating itself over and over again)(my theory is that since we are alive, we represent a chaos that will change the next collapse and expansion of the universe).