Enumerate with two columns

Question

I'd like to have an enumerate with two columns. The first one report the name of the property and the second one can be a multiline text and it should be indented correctly. Thus second columns' text should follow the document's margin.

Something like this:

\begin{enumerate}
\item[] \textbf{Diagonal}, if all its elements outside the main diagonal are equal to zero, that is $\forall i\neq j$, $a_{ij} = 0 $ 
\item[] \textbf{Scalar}, if all off-diagonal elements are zero and all on-diagonal elements are equal 
\item[] \textbf{Identity}, if it's scalar and the elements on its main diagonal are all equal to 1, that is $\forall i$, $a_{ii} = 1$ 
\item[] \textbf{Lower triangular}, if all the entries above the main diagonal are zero, that is $\forall i > j$, $a_{ij} = 0$ 
\item[] \textbf{Upper triangular}, if all the entries below the main diagonal are zero, that is $\forall i < j$, $a_{ij} = 0$
\end{enumerate}
Below, a brief summary of the matrix operations used:
\begin{enumerate}
\item[] \textbf{Addition}: If $A$ and $B$ are matrices of the same size then the sum $A$ and $B$ is defined by $C = A + B$, where 
\begin{equation}
c_{ij} = a_{ij}+b_{ij} \quad \forall i,j
\end{equation}
\end{enumerate}

Using leandriis' link, this is what I obtained

"Diagonal" has \quad\quad after it, and I would like to indent all the text in the second column "Scalar" is the normal result.

Answer 1

A longtable environment may be well suited for your typesetting needs.

You may see fit to adjust the width of the second column depending on (a) the width of the first column and (b) the overall width of the text block.

\documentclass{article}
\usepackage{array,longtable,ragged2e}
\newcolumntype{P}[1]{>{\RaggedRight\arraybackslash}p{#1}}

\begin{document}
\noindent
A matrix is said to be
\begin{longtable}{@{} >{\bfseries}l P{0.6\textwidth} @{}}
Diagonal & if all its elements outside the main diagonal are equal to zero, that is $\forall i\neq j$, $a_{ij} = 0 $\\
Scalar & if all off-diagonal elements are zero and all on-diagonal elements are equal\\
Identity & if it is scalar and the elements on its main diagonal are all equal to 1, that is, $\forall i$, $a_{ii} = 1$ \\
Lower triangular & if all the entries above the main diagonal are zero, that is, $\forall i > j$, $a_{ij} = 0$ \\
Upper triangular & if all the entries below the main diagonal are zero, that is, $\forall i < j$, $a_{ij} = 0$
\end{longtable}

\bigskip\noindent
Below, a brief summary of the matrix operations used:
\begin{longtable}{@{} >{\bfseries}l P{0.7\textwidth} @{}}
Addition & If $A$ and $B$ are matrices of the same size then the sum of $A$ and $B$ is defined by $C = A + B$, where
\begin{equation}
c_{ij} = a_{ij}+b_{ij} \quad \forall i,j
\end{equation}
\end{longtable}
\end{document}