Then the second derivative gives the change in entropy with respect to the variable that we're differentiating, with respect to which is either pressure or volume.

二阶导数给出熵,随着变量变化的情况，这些变量包括压强或者体积。

麻省理工公开课 - 热力学与动力学课程节选

Second good piece of style here is choice of variable names.

第二个编程优秀风格就是,变量名称的选择。

麻省理工公开课 - 计算机科学及编程导论课程节选