\begin{definition}[Definition:Differential of Mapping/Functional]
Let $J \sqbrk y$ be a differentiable functional.
Let $h$ be an increment of the independent variable $y$.
Then the term linear {{WRT}} $h$ is called the '''differential''' of the functional $J$, and is denoted by $\delta J \sqbrk {y; h}$.
\end{definition}