Small title for equation on the left and right margins

Question

I would like to display a small title for each equation on the left and right margins of a document like this

How to do it please?

Answer 1

The note in the margin can be added using the command \marginnote from marginnote, and the box around the equation can be added with the empheq environment from empheq.

\documentclass{article}
\usepackage{marginnote}
\usepackage{empheq}
\usepackage{xcolor}
\newcommand*{\bluebox}[1]{\fcolorbox{blue}{white}{#1}}
\reversemarginpar
\begin{document}
This equality implies that it makes sense to introduce the notion of the \emph{electric field} at point \(\mathbf{r}\) (as an entity independent of \(q\)), characterized by the following vector:
\begin{empheq}[box=\bluebox]{equation}
\marginnote{\color{blue}Electric field: definition}[-2mm]
\mathbf{E}(\mathbf{r}) \equiv \frac{\mathbf{F}(\mathbf{r})}{q},
\end{empheq}
formally called the \emph{electric field strength} -- but much more frequently, just the ``electric field''.
\end{document}

Answer 2

A combination of \boxed (a macro provided by the amsmath package) and \marginnote (a macro provided by the marginnote package) gets the job done.

\documentclass{article}
\usepackage{marginnote} % for '\marginnote' macro
\reversemarginpar
\usepackage{amsmath}    % for '\boxed' macro
\usepackage{xcolor}     % for '\color' macro

\begin{document}
This equality implies that it makes sense to define an \emph{electric field} 
at a point~\(\mathbf{r}\), as an entity that is 
independent of~\(q\), as:
\begin{equation}
\boxed{\mathbf{E}(\mathbf{r}) \equiv \frac{\mathbf{F}(\mathbf{r})}{q}\,.}
\marginnote{\color{blue}Electric field: definition}
\end{equation}
\(\mathbf{E}\) is formally called the \emph{electric field 
strength} or, far more frequently, just the ``electric field.''
\end{document}