dyet

An infinitesimal which is not the differential of an actual function and which cannot be expressed as d z = ((dz)/(dx))_y d x + ((dz)/(dy))_x d y, the way an exact differential can. Inexact differentials are denoted with a bar through the d. The most common example of an inexact differential is the change in heat d- Q encountered in thermodynamics.

exact differential | Pfaffian form