I am reading notes on finite model theory and I came across a notion of a vocabulary. What is it? I tried searching on the internet but did not find anything relevant.
See Leonid Libkin, Elements of Finite Model Theory, Springer (2012), page 13 :
Definition 2.1 A vocabulary $σ$ is a collection of constant symbols (denoted $c_1,\ldots, c_n,\ldots$), relation, or predicate, symbols ($P_1,\ldots,P_n,\ldots$) and function symbols ($f_1,\ldots,f_n,\ldots$). Each relation and function symbol has an associated arity.
Is the set of non-logical symbols of the language, also called signature.