Gottlob Frege, answered from the texts and cited to the page.
Every meaningful linguistic expression has two distinguishable semantic dimensions. The Sinn (sense) is the mode of presentation of what is designated; the Bedeutung (reference or meaning) is what is designated.1 For proper names, the Sinn is the way the bearer is presented (the morning star, the evening star are two senses of the same object); the Bedeutung is the bearer itself.2
For predicates and function-expressions, the Sinn is the predicate's mode of presentation; the Bedeutung is the concept or function it stands for. For sentences, the Sinn is the Gedanke (Thought) expressed; the Bedeutung is the truth-value (the True or the False). The distinction is required to solve the cognitive-significance puzzle: that a = a is trivial while a = b may be a genuine discovery though the two flank the same object.
The compositionality principle holds at both levels — the Sinn of a sentence is composed of the Sinne of its parts; the Bedeutung of a sentence is determined by the Bedeutungen of its parts.345
A difference can arise only if the difference between the signs corresponds to a difference in the mode of presentation of the thing designated. Let a, b, c be the lines connecting the vertices of a triangle with the midpoints of the opposite sides. The point of intersection of a and b is then the same as the point of intersection of b and c.Collected Papers on Mathematics, Logic, and Philosophy, p. 164
And the sense of the name 'morning star' is indeed different from that of the word 'evening star'. And so it happens that the thought of our first proposition is different from that of the second; for the thought we express in a prdposition is the sense of the proposition. This is probably one of the cases of which you say, in your notice of my Foundations of Arithmetic (Rivista di matematica, vol, V, p.Philosophical and Mathematical Correspondence, p. 146
The latter of course is essential only if we wish to operate in the realm of science. In the case of fiction it does not matter whether the people who occur in it are, as we should say, historical personages. Or speaking more precisely, 'whether the personal proper names occurring in fiction are meaningful'.Posthumous Writings, pp. 246–247
But we can immechately infer from what we have just said that something further must be associated wilt the proper name, something which is different from the object designated and which is essential to the thought of the sentence in which the proper name occurs. | call it the sense of the proper naine. As the proper ume is part of the sentence, so its sense art of the thought.Posthumous Writings, pp. 205–206
At any rate, one might expect that such sentences occur, just as there are parts of sentences having sense but no meaning. And sentences which contain proper names without meaning will be of this kind. The sentence 'Odysseus was set ashore at Ithaca while sound asleep' obviously has a sense.Collected Papers on Mathematics, Logic, and Philosophy, pp. 168–169
The founding distinction of modern philosophy of language and semantics. Russell's theory of descriptions (1905) is a direct response. Carnap, Church, Geach, and Dummett develop the distinction in various directions. Saul Kripke's Naming and Necessity (1980) is the canonical challenge: proper names are rigid designators, not descriptive senses; the modal argument and the epistemic argument show that the descriptive content one associates with a name is not the meaning of the name. The post-Kripke direct-reference school (Donnellan, Kaplan, Salmon, Soames) generalizes the line. The neo-Fregean response (Evans, McDowell, Burge, Peacocke) preserves the Sinn/Bedeutung distinction for indexicals, demonstratives, and the cognitive-significance role, even where Kripke's rigid-designator point is granted for typical proper names.