Vacuum is space void of matter. The word stems from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum" or free space, and use the term partial vacuum to refer to an actual imperfect vacuum as one might have in a laboratory or in space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is lower than atmospheric pressure. The Latin term in vacuo is used to describe an object as being in what would otherwise be a vacuum.

The quality of a partial vacuum refers to how closely it approaches a perfect vacuum. Other things equal, lower gas pressure means higher-quality vacuum. For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%. Much higher-quality vacuums are possible. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth (1012) of atmospheric pressure (100 nPa), and can reach around 100 particles/cm3. Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average. According to modern understanding, even if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics. In the electromagnetism in the 19th century, vacuum was thought to be filled with a medium called aether. In modern particle physics, the vacuum state is considered the ground state of matter.

1. What does a vacuum not have?
2. Where does the word come from?
3. What language is that?
4. what does it mean?
5. Is that an adverb?
6. What is it?
7. What do physicists call vacuum?
8. What else?
9. What do they call a vacuum that's not perfect?
10. How does pressure in a vacuum compare to pressure in the atmosphere?
11. What does a household vacuum do to air pressure?
12. by how much?
13. Is outer space a vacuum?
14. How many hydrogen atoms or in a cubic meter out there?
15. would something be empty if there was no matter in it?
16. Why not?
17. Is vacuum one of them?
18. What's another one?
19. What did they once believe was in a vacuum?
20. When did they think this?

Provide a numbered list of answers.
1. matter
2. vacuus
3. Latin
4. vacant or void
5. no
6. adjective
7. a perfect vacuum
8. free space
9. partial vacuum
10. it's lower
11. reduces it
12. 20 percent
13. yes
14. just a few
15. no
16. phenomena in quantum physics
17. yes
18. cosmic rays
19. aether
20. in the 19th century