Describes any pressure less than atmospheric.

Hard vacuum is defined as less than one Torr (one millimeter of mercury) and it can present a problem for the elastomer (O-ring) in many seal applications The vacumm can pull air out of the elastomer, cause it to lose volume and leak the fluid it is sealing

Vacuum is the same as “nothing”. If you want to see nothing, pretend you have two eyes in the back of your head and they’re blind. What do you see?

That’s nothing!

Please see: Torr, T019