\begin{symboleqn}{(*)}
x^n + y^n = z^n.
\end{symboleqn}
Any text can replace the (*), but keep in mind that it will be set in
math mode. Equations generated by symboleqn cannot be labeled;
any references must be constructed explicitly.
Since asterisks are a favorite symbol in this connection, let me note
some of the pitfalls. Always set (*) in math mode thus:
$(*)$. If you enter (*), you will get (*), with the
asterisk too high. To get (***) enter $(*{*}*)$. The
braces keep TEX from treating the inner asterisk as an operator.