\begin{definition}[Definition:Sign (Mathematical Theory)]
A '''sign''' of a mathematical theory is a symbol which has a specific meaning in the theory, as follows:
# A logical sign;
# A letter;
# A specific sign.
A logical sign is a sign which has a specific meaning and definition in a particular class of mathematical theory.
A letter is a more or less arbitrary symbol whose definition depends on the specific context.
For a given mathematical theory, there are signs which are specific to the use to which the theory is being put. These are the specific signs.
{{wtd|This may need to go, as it covers the same ground as Sign (Formal Systems).}}
\end{definition}