\documentclass{minimal}
\begin{document}

\begin{defn}[Definition 1]
  \label{defn:definition_1}
  A simple environment with simple contents.
\end{defn}

\begin{figure}[ht] % A standalone figure with label and caption
  \centering
  %\input{proof}
  \caption{Simple figure}
  \label{fig:simple_figure}
\end{figure}

\begin{thm}[Example Title of Theorem]
  \label{thm:example_title_of_theorem}
  Suppose A and B.

  \begin{itemize}
    \item If C, then D
    \item \(A \wedge B \implies D \iff A \wedge B \implies C\)
  \end{itemize}

  We demonstrate this by the figure.

  \begin{figure}[ht] % Comment
    \centering
    %\input{proof}
    \caption{A visual proof of Example Theorem}
    \label{fig:visual_theorem}
  \end{figure}
\end{thm}

\begin{defn}[Definition 2]
  \label{defn:definition_2}
  A sample definition.

  \caption{A caption for an environment typically without caption}
\end{defn}

\end{document}