Reference.. National Physical Laboratory
Pressure is generally the result of molecules, within a gas or liquid, impacting on their surroundings - usually the walls of the containing vessel. Its magnitude depends on the force of the impacts over a defined area; hence, for example, the Newton per square metre, given the special name pascal, and the traditional (but obsolete) unit pounds force per square inch.
This equation applies whether the pressure is very small, such as in outer space, or very large, as in hydraulic systems for example. Thus the word pressure is correct when referring to the entire range of 'force per unit area' measurements, although at extremely low pressures the concept of molecules exerting a force becomes more abstract.
Its definition is not precise but, as mentioned in the section above, it is commonly taken to mean pressures below, and often considerably below, atmospheric pressure. It does not have separate units and we do not say that 'vacuum equals force per unit area'.
Thus, strictly, we do not need to talk about both pressure and/or vacuum because vacuum is pressure. But the differences are often misunderstood and thus leaving out the word vacuum can falsely imply that the pressure in question is above that of atmospheric pressure.
Another definition of the distinction between pressure and vacuum comes from the industries which use and make pressure and vacuum equipment.
Broadly, if the force on the walls of the containing vessel is sufficient to permit its measurement directly, we are dealing with pressure technology but if the force is too small for direct measurement and has to be indirectly inferred, we are in the realm of vacuum technology.
This definition is not entirely self-consistent though; for example there is a class of instrument which operates in the vacuum region by measuring the deflection of a diaphragm.
You probably already know how this one ends, but that doesn't make watching it play out any less spectacular.
It was Galileo himself who first discovered that in a vacuum, if you were to drop two objects from the same height, they'd hit the ground at exactly the same time, regardless of their respective weights. Of course, on Earth, we rarely - if ever - get the change to see this at play, thanks to a phenomenon known as air resistance.
The combination of bowling ball and feather is the perfect way to demonstrate air resistance, also known as drag. Because the shape of the feather allows it to endure way more air resistance than the bowling ball, it takes much longer to fall to the ground.
British physicist Brian Cox wanted to see this primary-school problem play out in a vacuum, where there is zero air resistance to mess with the results. Filming for his new BBC 2 show, Human Universe, he travelled to the US and visited the NASA Space Power Facility in Ohio. The facility is the world's largest vacuum chamber, measuring 30.5 metres by 37.2 metres, and has a volume of 22,653 cubic metres.
When not in use, the chamber contains around 30 tonnes of air, but when it's turned on, all but around 2 grams of air are sucked out to create an artificial vacuum. Watch above to see what happens when a bowling ball and feather are dropped in the chamber under 'normal' conditions and then in a vacuum. If it's enough to make even the most seasoned NASA scientist grin with childlike wonder, you know it's gotta be good.