The link produced by \ref{defn:First Defined Term}
works fine, but
as I would prefer to have text instead of a number so thought
\href{defn:First Defined Term}{First Defined Term}
should work, but
it doesn't:
\documentclass{article}
\usepackage{thmtools}
\usepackage{hyperref}
\usepackage{lipsum}
\declaretheorem[name=Definition]{DefinitionTheorem}
\newenvironment{Definition}[1]{%
\begin{DefinitionTheorem}
\label{defn:#1}
}{%
\end{DefinitionTheorem}
}
\begin{document}
\lipsum[1-7]
\begin{Definition}{First Defined Term}
\lipsum[1]
\end{Definition}
\lipsum[8-10]
\begin{Definition}{Second Defined Term}
\lipsum[1]
\bigskip
This link works great: \ref{defn:First Defined Term}\par\bigskip
But this link does not: \href{defn:First Defined Term}{First Defined Term}
\end{Definition}
\end{document}