Sentence:  If * * f is the change per unit volume in Gibbs <WSD>function</WSD> caused by the shear field at constant P and T , and |r is the density of the fluid , then the total potential energy of the system above the reference height is * * f . 
Meaning of the word marked with <WSD> tags:  (mathematics) a mathematical relation such that each element of a given set (the domain of the function) is associated with an element of another set (the range of the function)