The Commens Dictionary

Quote from ‘Firstness, Secondness, Thirdness, and the Reducibility of Fourthness [R]’

…a sign may, in its secondness to the object as represented, [—] either, as an ‘Icon,’ be related to that object by virtue of a character which belongs to the sign in its own firstness, and which equally would belong to it though the object did not exist, or, as an ‘Index,’ [it] may be related to the object by a real secondness, such as a physical connection […] to it, or it may, as a ‘Symbol,’ be related to its object only because it will be represented in its interpretant as so related, as is the case with any word or other conventional sign, or any general type of image regarded as a schema of a concept.