Inexact Differential

An infinitesimal which is not the differential of an actual function and which cannot be expressed as

$$dz=\left({\partial z\over\partial x}\right)_y\,dx+\left({\partial z\over\partial y}\right)_z\,dy,$$

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 ${\mathchar'26\mkern-12mud}Q$ encountered in thermodynamics.

See also Exact Differential, Pfaffian Form

References

Zemansky, M. W. Heat and Thermodynamics, 5th ed. New York: McGraw-Hill, p. 38, 1968.