Split text nodes multiline in tikz-cd

You can for example use a \parbox or tabular for the long labels:

\documentclass{article}
\usepackage{tikz-cd}
\usepackage{amsmath}

\begin{document}
\begin{tikzcd}[row sep=3cm, column sep=0.9cm]
& \parbox{2cm}{\centering binary relations}
\arrow[rd, shift left]
\arrow[ld, shift left] \\
\parbox{3cm}{\centering pointfree funcoids between powersets}
\arrow[ru, shift left]
\arrow[rr, shift left]
& & \parbox{3cm}{\centering antitone Galois connections between powersets}
\arrow[lu, shift left]
\arrow[ll, shift left]
\end{tikzcd}

\begin{tikzcd}[row sep=3cm, column sep=0.9cm,every cell/.append style={align=center}]
& \text{binary relations}
\arrow[rd, shift left]
\arrow[ld, shift left] \\
\begin{tabular}{c}
pointfree funcoids \\
between powersets
\end{tabular}
\arrow[ru, shift left]
\arrow[rr, shift left]
& & \begin{tabular}{c}
antitone Galois connections \\
between powersets
\end{tabular}
\arrow[lu, shift left]
\arrow[ll, shift left]
\end{tikzcd}
\end{document}

Here's a solution:

\documentclass{article}
\usepackage{tikz-cd}
\usepackage{amsmath}
\newcommand\mlnode[1]{\fbox{\begin{tabular}{@{}c@{}}#1\end{tabular}}}

\begin{document}

\begin{figure}[ht]
  \begin{tikzcd}[row sep=3cm, column sep=0.9cm]
    & \fbox{binary relations}
    \arrow[rd, shift left]
    \arrow[ld, shift left] \\
    \mlnode{pointfree funcoids\\ between powersets}
    \arrow[ru, shift left]
    \arrow[rr, shift left]
    & & \mlnode{antitone Galois\\ connections\\ between powersets}
    \arrow[lu, shift left]
    \arrow[ll, shift left]
  \end{tikzcd}
\end{figure}

\end{document}