In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.
that's a copy and paste.
Gasses are fluids, but gasses are not liquids.
IIRC from "high school", Liquids assume the shape of the vessel holding them to the level of fill.
Gasses FILL the vessel that contains them.
Both are fluids.
With that, a pinhole leak can send out a knife of a stream of liquid that will penetrate skin as easily as a razor.
Be careful, use hydraulic fittings and common sense. Typical automotive brake line systems run around 1000 psi. Not a good match for a 4ksi application, but better than plumbing fittings.
Oh! I hand pump my PCPs, it's good exercise and keeps me mindful of where and when I pull the trigger. ;-)