Pressure

Pressure is a feature of a material whose value is what a gauge reads when the material pushes against a known surface. Like temperature, its value belongs to the condition of the material, not to its amount. A small flask of gas and a large flask of gas can sit at the same pressure, and the gauge will say so for both.

Pressure belongs to the core features of a phase, alongside temperature, volume, mass, energy, and the others. It joins temperature as one of the conditions a system can be in, and like temperature it is intensive. States of a system can differ in their values of pressure and temperature, and we describe a system's condition by giving both. Two samples at the same pressure and the same temperature are, in those respects, sitting in the same condition, regardless of their size.

Notice the parallel construction. Temperature is what a thermometer reads. Pressure is what a gauge reads. We do not, in this approach, reach for a story about what is happening below the surface to "explain" what either reading means. The reading is the value, the value is what the property holds, and the property is how we describe the material. The discipline is to stay at the level where we can actually look.

Pressure also drives things to happen. Where pressure is unequal across two parts of a system, material moves: from higher pressure toward lower, until the values balance. A balloon pinched at the neck releases when you let go, and the gauge reading on each side moves toward the other. As with temperature and density, the unevenness of pressure is part of the story of change.