That which brings something else into the ambit of a knowing power.  According to a number of scholastics, a sign is a relation (relatio secundum esse), not the visible vehicle by which a sign is notified to a knower.  Signs are divided into speculative and practical depending on the content of signification and the intellectual focus derived therefrom.  They are divided as instrumental and formal depending on whether they respectively do (or do not) require one to first know the sign in order to then pass on to what is signified.  In the unique case of the expressed species or concept formed by the internal senses and the intellect, the “formal sign” immediately signifies the known reality rendered present in that sign (as a medium in quo).  Signs exercise vicarious objective causality, which is a subdivision of extrinsic formal causality.  Semiotics is the study of the action of signs.