A sign in general is 1st something Real, that is 2nd applicable to an object different from itself and already known to the person to whom it is a sign, and 3rd is capable of interpretation in the mind of that person, so that it will (or would if accepted as veracious sign) have some effect upon him of a kind it was calculated or fit to have.
http://www.commens.org/dictionary/entry/quote-letters-paul-carus-8