A collection of Peirce's definitions of the sign are available at a different location. Therefore, this page offers only four samples of Peirce's characterisations of "sign". Please see 76 Definitions of The Sign by C. S. Peirce, with 12 Further Definitions or Equivalents (compiled by Robert Marty and Alfred Lang) for the longer list.

"A Sign is a Cognizable that, on the one hand, is so determined (i.e., specialized, bestimmt) by something other than itself, called its Object [...], while, on the other hand, it so determines some actual or potential Mind, the determination whereof I term the Interpretant created by the Sign, that that Interpreting Mind is therein determined mediately by the Object." (A Letter to William James, EP 2:492, 1909)

"I define a Sign as anything which is so determined by something else, called its Object, and so determines an effect upon a person, which effect I call its Interpretant, that the latter is thereby mediately determined by the former. My insertion of "upon a person" is a sop to Cerberus, because I despair of making my own broader conception understood." (A Letter to Lady Welby, SS 80-81, 1908)

"A sign, or representamen, is something which stands to somebody for something in some respect or capacity. It addresses somebody, that is, creates in the mind of that person an equivalent sign, or perhaps a more developed sign. That sign which it creates I call the interpretant of the first sign. The sign stands for something, its object. It stands for that object, not in all respects, but in reference to a sort of idea, which I have sometimes called the ground of the representamen. "Idea" is here to be understood in a sort of Platonic sense, very familiar in everyday talk; I mean in that sense in which we say that one man catches another man's idea, in which we say that when a man recalls what he was thinking of at some previous time, he recalls the same idea, and in which when a man continues to think anything, say for a tenth of a second, in so far as the thought continues to agree with itself during that time, that is to have a like content, it is the same idea, and is not at each instant of the interval a new idea. (A Fragment, CP 2.228, c. 1897)

"For the purposes of this inquiry a Sign may be defined as a Medium for the communication of a Form. It is not logically necessary that anything possessing consciousness. that is, feeling of the peculiar common quality of all our feeling should be concerned. But it is necessary that there should be two, if not three quasi-minds, meaning things capable of varied determination as to forms of the kind communicated." ([On Signs], MS 793:1, not dated)

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Abbreviations (CP, EP, etc.) and sources; see here