Use a regular enumerate
and set every equation inside the same-sized box (using eqparbox
's \eqmakebox[<tag>][<align>]{<stuff>}
:

\documentclass{article}
\usepackage{eqparbox}
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed ut lorem ac orci ullamcorper mattis.
Nunc at convallis turpis, aliquam vehicula leo. Etiam mollis, urna nec interdum suscipit, lacus
ante mollis risus, placerat efficitur lacus quam at mauris. Vestibulum pretium dolor a eleifend
vestibulum. Duis sed ornare sapien.
\begin{enumerate}
\item First item
\item Second item
\item Third item
\end{enumerate}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed ut lorem ac orci ullamcorper mattis.
Nunc at convallis turpis, aliquam vehicula leo. Etiam mollis, urna nec interdum suscipit, lacus
ante mollis risus, placerat efficitur lacus quam at mauris. Vestibulum pretium dolor a eleifend
vestibulum. Duis sed ornare sapien.
\begin{enumerate}
\item
\eqmakebox[leq][l]{$f(x) = \Theta$} \qquad (belonging to~$\Theta$)
\item
\eqmakebox[leq][l]{$f(x+y) = x \leftrightarrow x > y$} \qquad (identity of confusion)
\item
\eqmakebox[leq][l]{$\lambda f(x) = f(x^\lambda)$} \qquad (non-sensible relation)
\end{enumerate}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed ut lorem ac orci ullamcorper mattis.
Nunc at convallis turpis, aliquam vehicula leo. Etiam mollis, urna nec interdum suscipit, lacus
ante mollis risus, placerat efficitur lacus quam at mauris. Vestibulum pretium dolor a eleifend
vestibulum. Duis sed ornare sapien.
\end{document}
Using a list instead of a tabular
will ensure things line up the same way your other lists do, and setting the content in similarly-sized boxes will ensure horizontal alignment with your "comments."