Mathematical notation in latex

Question

How can I write in latex the $=$ symbol with the word 'def' above it??!

It is because Ive to make use of a definition.

Answer 1

\stackrel{\mathrm{def}}{=} produces your desired $\stackrel{\text{def}}{=}$

Answer 2

If AMSMATH package has been loaded, is to use "\overset":

Example.

\documentclass{article}
\usepackage{amsmath}

\newcommand\myeq{\mathrel{\overset{\makebox[0pt]  
{\mbox{\normalfont\tiny\sffamily def}}}{=}}}

\begin{document}

\begin{align*}
a &\myeq b \\
  &=c \\
  &= d.
\end{align*}

\end{document}