\begin{definition}[Definition:Signature (Logic)/Predicate Logic]
Let $\LL_1$ be the language of predicate logic.
Then a '''signature''' for $\LL_1$ is an explicit choice of the alphabet of $\LL_1$.
That is to say, it amounts to choosing, for each $n \in \N$:
:A collection $\FF_n$ of $n$-ary function symbols
:A collection $\PP_n$ of $n$-ary relation symbols
It is often conceptually enlightening to explicitly address the $0$-ary function symbols separately, as constant symbols.
\end{definition}