\documentclass{IEEEtran}
\newtheorem{definition}{{\bf Definition}}
\newcommand{\qed}{\nobreak \ifvmode \relax \else
 \ifdim\lastskip<1.5em \hskip-\lastskip
  \hskip1.5em plus0em minus0.5em \fi \nobreak
  \vrule height0.75em width0.5em depth0.25em\fi}
\begin{document}
\begin{definition}
Here is a definition.
\end{definition}
\end{document}