This element of our daily & hourly experience, the element of the conformity of fact to thought, – this element whose being such as it is consists in this that it has such reference to an object independent of it as to bring a third thing (the interpretation) into the same triadic relation to that same object, – this character of a sign, the being an exponent of thought, is what I call the element of Thirdness in the phenomenon.
http://www.commens.org/dictionary/entry/quote-csps-lowell-lectures-1903-2nd-draught-3rd-lecture-6