Exact differential
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 d Q {\displaystyle dQ} for some differentiable function Q {\displaystyle Q} in an orthogonal coordinate system (hence Q {\displaystyle Q} 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.