\documentclass{article}

\begin{document}

\begin{definitions}
  Here is a code  chunk.
\begin{definition}

<<foo, fig.height=4>>=
    aa <- "sdfdsf"
    plot(rnorm(100))
@

\end{definition}

\end{definitions}
You can also write inline expressions, e.g. $\pi=\Sexpr{pi}$, and
\Sexpr{1.598673e8} is a big number.

\end{document}