Definition
Sense and reference is Gottlob Frege's distinction between two aspects of a linguistic expression's meaning. The reference (Bedeutung) is the object in the world that the expression picks out — the actual planet Venus, for instance. The sense (Sinn) is the mode of presentation through which it picks that object out — the cognitive route by which a speaker identifies it. "The morning star" and "the evening star" share a reference (both name Venus) but differ in sense (one presents Venus as the brightest body in the dawn sky, the other in the dusk sky).
Frege introduced this distinction in his 1892 paper "On Sense and Reference" to solve a puzzle about identity statements. If "the morning star is the evening star" merely linked two names of the same object, it would be as trivially true as "Venus is Venus." Yet the identity was an empirical discovery — informative, not trivial. The asymmetry shows that names contribute more to a sentence's content than the bare object they refer to: they contribute a sense.
Why it matters
How it works
Every meaningful expression, on Frege's account, is associated with both a sense and a reference. For a singular term like a proper name, the reference is the object named and the sense is the descriptive content by which a competent speaker grasps which object that is. For a complete declarative sentence, the reference is its truth value (true or false, treated as abstract objects) and the sense is the thought it expresses. This treatment is unusual, but it lets Frege preserve compositionality — the principle that the meaning of a whole is determined by the meanings of its parts — at both levels: the sense of a sentence is composed from the senses of its parts, and the reference of a sentence is composed from the references of its parts.
The distinction also explains why substituting co-referring terms preserves reference but not always sense. In an extensional context like "the morning star has a thick atmosphere," replacing "the morning star" with "the evening star" preserves truth value because both phrases refer to Venus. In an intensional or opaque context like "Ada believes the morning star is visible at dawn," the same substitution can flip the truth value, because Ada may not know the two phrases pick out the same object. What the report tracks is the sense Ada grasps, not the reference she would acknowledge if informed.