In logic, the formal languages used to create expressions consist of symbols, which can be broadly divided into constants and variables.
The constants of a language can further be divided into logical symbols and non-logical symbols (sometimes also called logical and non-logical constants).
The non-logical symbols of a language of first-order logic consist of predicates and individual constants.
These include symbols that, in an interpretation, may stand for individual constants, variables, functions, or predicates.
The latter include logical connectives, quantifiers, and variables that stand for statements.
These concepts are defined and discussed in the article on first-order logic, and in particular the section on syntax.
The logical constants, by contrast, have the same meaning in all interpretations.
The additional information controls how the non-logical symbols can be used to form terms and formulas.
An example of a structure over the signature mentioned above is the ordered group of integers.
Rudolf Carnap introduced a terminology distinguishing between logical and non-logical symbols (which he called descriptive signs) of a formal system under a certain type of interpretation, defined by what they describe in the world.
A descriptive sign is defined as any symbol of a formal language which designates things or processes in the world, or properties or relations of things.
This is in contrast to logical signs which do not designate any thing in the world of objects.
The use of logical signs is determined by the logical rules of the language, whereas meaning is arbitrarily attached to descriptive signs when they are applied to a given domain of individuals.