Massieu function

In thermodynamics, Massieu function, symbol ${\displaystyle \Psi }$ (Psi), is defined by the following relation:

where for every system with degree of freedom r one may choose r variables, e.g. ${\displaystyle {\big (}X_{1},\dots ,X_{i},Y_{i+1},\dots Y_{r}{\big )}\,}$, to define a coordinate system, where X and Y are extensive and intensive variables, respectively, and where at least one extensive variable must be within this set in order to define the size of the system. The (r + 1)-th variable, ${\displaystyle \Psi }$, is then called the Massieu function.

...
Wikipedia