I think this article is written a bit too narrowly, but I'm not exactly sure how to go about changing it. One thing that is certainly true is that the concept of "PSI" does not imply a confining vessel or enclosure. That is only one possible application of this concept. I think I would start by writing this (corrections are welcome):
PSI is an acronym that stands for "pounds per square inch." That is, force per unit area where the force is expressed in units of pounds? and the area is expressed in units of square inches.
-- TrickyP?
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