Why do we refer to pressure unequilibrium as partial vaccuum?

Question

I was reading about Newcomen’s engine where the introduction of water to the cylinder makes the steam go into liquid, and they refer to it as “partial vaccuum” creation. How come?

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Answer 1

An actual vacuum contains nothing. Like, literally no matter in it at all. A partial vacuum has very little in it, but does have some matter. For reference, space is not a perfect vacuum, as it contains a few hydrogen atoms per cubic metre, so the barrier for what is a perfect vacuum is pretty absolute