Since you're loading the amsmath
package, you could make use of that package's \tagform@
macro to achieve your objective.
Note that you will not be able to use \eqref
to create cross-references to equations; I suggest you load the cleveref
package and use \cref
instead.

\documentclass{scrbook}
\usepackage{amsmath,amssymb,amsthm}
\makeatletter
\def\tagform@#1{\maketag@@@{\ignorespaces(Equation #1\unskip\@@italiccorr)}}
\makeatother
\usepackage[noabbrev,capitalize]{cleveref}
\begin{document}
\setcounter{chapter}{1}
\begin{equation}
\label{eqn:example}
a+b=c
\end{equation}
A cross-reference to \cref{eqn:example}.
\end{document}
Addendum: If you're willing to load the mathtools
package (which automatically loads amsmath
), you could make use of the \newtagform
macro. (I'm not posting a second screenshot, as the output of the following example looks just like the one above.)
\documentclass{scrbook}
\usepackage{mathtools,amssymb,amsthm}
\newtagform{Eq}{(Equation }{)}
\usetagform{Eq}
\usepackage[noabbrev,capitalize]{cleveref}
\begin{document}
\setcounter{chapter}{1}
\begin{equation}
\label{eqn:example}
a+b=c
\end{equation}
A cross-reference to \cref{eqn:example}.
\end{document}
\ref{eqn:example}
also returnEquation X.Y
? Are you usinghyperref
?