The Commens Dictionary
Quote from ‘Letters to Lady Welby’
In respect to their relations to their dynamic objects, I divide signs into Icons, Indices, and Symbols (a division I gave in 1867). I define an Icon as a sign which is determined by its dynamic object by virtue of its own internal nature. Such is any qualisign, like a vision, - or the sentiment excited by a piece of music considered as representing what the composer intended. Such may be a sinsign, like an individual diagram; say a curve of the distribution of errors.