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 (10−12) 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.
What does a vacuum not have?
Where does the word come from?
What language is that?
what does it mean?
Is that an adverb?
What is it?
What do physicists call vacuum?
What else?
What do they call a vacuum that's not perfect?
How does pressure in a vacuum compare to pressure in the atmosphere?
What does a household vacuum do to air pressure?
by how much?
Is outer space a vacuum?
How many hydrogen atoms or in a cubic meter out there?
would something be empty if there was no matter in it?
Why not?
Is vacuum one of them?
What's another one?
What did they once believe was in a vacuum?
When did they think this?
matter
Vacuum is space void of matter
vacuus
The word stems from the Latin adjective vacuus for "vacant" or "void"
Latin
the Latin adjective vacuus 
vacant or void
 vacuus for "vacant" or "void"
no
the Latin adjective vacuus 
adjective
 the Latin adjective vacuus
a perfect vacuum
Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call "vacuum"
free space
which they sometimes simply call "vacuum" or free space
partial vacuum
the term partial vacuum to refer to an actual imperfect vacuum 
it's lower
he pressure is lower than atmospheric pressure. 
reduces it
a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%
20 percent
 a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%
yes
Outer space is an even higher-quality vacuum
just a few
 just a few hydrogen atoms per cubic meter on average
no
even if all matter could be removed from a volume, it would still not be "empty"
phenomena in quantum physics
 due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics
yes
it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics.
cosmic rays
vacuum fluctuations, dark energy, transiting gamma rays, cosmic rays, neutrinos, and other phenomena in quantum physics.
aether
 in the 19th century, vacuum was thought to be filled with a medium called aether
in the 19th century
in the 19th century, vacuum was thought to be filled with a medium called aether.