Short answers, pulled from the story.
The SI unit of pressure is the pascal (Pa), equal to one newton per square metre. The name was added to the SI system in 1971; before that, the same quantity was expressed simply as newtons per square metre.
Absolute pressure is measured relative to a perfect vacuum, while gauge pressure is measured relative to the ambient atmospheric pressure. A car tyre described as inflated to 220 kPa gauge pressure actually contains air at about 320 kPa absolute, because atmospheric pressure at sea level adds roughly 100 kPa.
Pressure in a liquid is determined by the weight of liquid directly above a given point, which depends only on depth and density. A swimmer one metre below the surface of a small pool feels the same pressure as a swimmer one metre below the surface of a large lake.
Liquid mercury has been observed sustaining a negative absolute pressure of -425 (in the relevant units) in clean glass containers without undergoing cavitation.
Blaise Pascal and Daniel Bernoulli are the two figures most credited with the foundational discoveries in fluid pressure. The SI unit the pascal is named after Pascal, and Bernoulli's equation is used to determine pressure at any point in a fluid.
Generalized pressure is a concept proposed in a 2022 paper in Physical Review B by the Lei Li group at Sichuan University. It describes a thermodynamic framework that treats different sources of volumetric strain, such as chemical and mechanical pressure, as equivalent when they produce the same volume change, drawing on Einstein's equivalence principle.