text: In multivariate calculus, a differential or differential form is said to be exact or perfect (exact differential), as contrasted with an inexact differential, if it is equal to the general differential  for some differentiable function  in an orthogonal coordinate system (hence  is a multivariable function whose variables are independent, as they are always expected to be when treated in multivariable calculus).
  An exact differential is sometimes also called a total differential, or a full differential, or, in the study of differential geometry, it is termed an exact form.
  The integral of an exact differential over any integral path is path-independent, and this fact is used to identify state functions in thermodynamics.