\input amstex

\def\dd{{\mathrm{d}}}
\def\defeq{\overset\hbox{\scriptsize def}\to=}

$$\displaystyle
\frac{\dd y}{\dd x}\defeq\lim\limits_{\fbox{$\scriptstyle\Delta x\rightarrow0$}}\frac{\Delta y}{\Delta x}%
=\lim\limits_{\fbox{$\scriptstyle\Delta x\rightarrow0$}}\frac{f(x+\Delta x)-f(x)}{\Delta x},\qquad y=f(x),\quad \Delta x=\dd x,\quad\Delta y=f(x+\Delta x)-f(x)$$