\begin{frame}
  \small
  
  \begin{definitions}
  \smallskip
  \begin{itemize}
  \item
    \makebox[1cm][l]{\alert<1>{SN}}
    \alert<1>{strong normalization} \quad or\quad
    \alert<1>{termination}
    \begin{itemize}
  \item
    no infinite rewrite sequences
    \end{itemize}
    \medskip
    \item<2->
    \makebox[1cm][l]{\alert<2,3>{WN}}
    \alert<2,3>{weak normalization}
    \begin{itemize}
  \item
    every element has (rewrites to) at least one normal form
    \smallskip
    \item<3->
    $\forall a$ $\exists b$ \, $a \to^! b$
    \end{itemize}
  \end{itemize}
  \smallskip
  \end{definitions}
  
\end{frame}