Remarks on the notion of “pressure”

• The different contexts in which the term pressure is used are discussed in detail.
• It is shown that the concept is incorrectly described in most texts dealing with the subject.
• A clear discussion of the differences between the various ways in which it is used is provided.

In addition to understanding the various meanings attached to the word “pressure” one also has to comprehend the meanings of the phrases in which the term “pressure” appears. For instance one comes across the following combinations: “static-fluid pressure”, “thermodynamic pressure”, “mechanical pressure”, “contact pressure”, “stagnation pressure”, “vapor pressure”, “electro-osmotic pressure”, etc., One also often comes across the comment that “pressure is the Lagrange multiplier that enforces the constraint of incompressibility” and that “pressure is the mean normal stress”. In general the word “pressure” with different significations, is used with gay abandon without paying proper attention to its usage1. The distinction in the meanings of the above terms assumes paramount significance when discussing properties of materials, which could possibly depend on “pressure”. In this short note we discuss the distinction between various significations of the word “pressure”, and their implications with regard to response relations for bodies.