Logical constant

Logical constants are those expressions within a logical system which determine the class of valid arguments of that system. The logical constants of propositional logic are the truth-functional connectives. The logical constants of predicate logic add to this set the quantifiers. By adding more logical constants to this set (like the identity sign '=', or tense operators) more complicated logical systems can be studied, allowing a larger class of valid arguments.

Link
Utrecht Lexicon of Linguistics