The ‘reference’ of an expression is the entity the expression designates or applies to. The ‘sense’ of an expression is the way in which the expression presents that reference. For example, the ancients used ‘the morning star’ and ‘the evening star’ to designate what turned out to be the same heavenly body, the planet Venus. These two expressions have the same reference, but they clearly differ in that each presents that reference in a different way. So, although coreferential, each expression is associated with a different ‘sense’. The distinction between sense and reference helps explain the cognitive puzzle posed by identity statements. ‘The morning star is the evening star’ and ‘The morning star is the morning star’ are both true, yet the sentences differ in cognitive significance, since the former may be informative, whereas the latter definitely is not. That difference in cognitive significance cannot be explained just by appeal to the references of the terms, for those are the same. It can, however, be naturally accounted for by appeal to a difference in sense. The terms ‘the morning star’ and ‘the evening star’ used in the first sentence, having different senses, present the referent in different ways, whereas no such difference occurs in the second sentence.
The distinction between sense and reference applies to all well-formed expressions of a language. It is part of a general theory of meaning that postulates an intermediate level of sense between linguistic terms and the entities the terms stand for. Senses give significance to expressions, which in and of themselves are just noises or marks on a surface, and connect them to the world. It is because linguistic terms have a sense that they can be used to express judgments, to transmit information and to talk about reality.