Semantics
Reference and sense
The reference of a word is the relation between the linguistic expression and the entity in the real world to which it refers. In contrast to reference, sense is defined as its relations to other expressions in the language system. Thus, there are words that have a sense, but no referents in the real world. Other words may differ in sense, but not necessarily in reference, and vice versa.
The class of entities to which an expression can be applied is usually called its extension. Consequently, the referent of a word is always a member of the class of entities that constitutes its extension. The word's intension, on the other hand, is defined as the set of semantic properties which define it.
The term denotation (that is also frequently used in the sense of an extensional reference) refers to the constant, abstract, and basic meaning of a linguistic expression. Secondary meanings or associations the expression evokes are called connotations.
To sum up, the terms reference, extension, and denotation relate to extra-linguistic reality, while the terms sense, intension, and connotation presuppose a language-internal definition of meaning.
Exercises on reference and sense