A representamen, or sign, is anything (not necessarily real) which stands at once in a relation of correspondence to a second third, its object, and to another possible representamen, its interpretant, which it determines to correspondence with the same object. It thus involves the idea of a possible endless series.
It is unclear whether the clarification in parenthesis should be placed after "anything" or "second third"
http://www.commens.org/dictionary/entry/quote-definitions-baldwins-dictionary-r-13