In calculus, the differential represents the principal part of the change in a function y = f(x) with respect to changes in the independent variable. The differential dy is defined by
d
y
=
f
′
(
x
)
d
x
,
{\displaystyle dy=f'(x)\,dx,}
where
f
′
(
x
)
{\displaystyle f'(x)}
is the derivative of f with respect to x, and dx is an additional real variable (so that dy is a function of x and dx). The notation is such that the equation
d
y
=
d
y
d
x
d
x
{\displaystyle dy={\frac {dy}{dx}}\,dx}
holds, where the derivative is represented in the Leibniz notation dy/dx, and this is consistent with regarding the derivative as the quotient of the differentials. One also writes
d
f
(
x
)
=
f
′
(
x
)
d
x
.
{\displaystyle df(x)=f'(x)\,dx.}
The precise meaning of the variables dy and dx depends on the context of the application and the required level of mathematical rigor. The domain of these variables may take on a particular geometrical significance if the differential is regarded as a particular differential form, or analytical significance if the differential is regarded as a linear approximation to the increment of a function. Traditionally, the variables dx and dy are considered to be very small (infinitesimal), and this interpretation is made rigorous in non-standard analysis.