A sign is $$something which stands to somebody for something,$$ and is categorized into three main types: $(1)$ an icon, which resembles its referent $($such as a road sign for falling rocks$)$; $(2)$ an index, which is associated with its referent $($as smoke is a sign of fire$)$; and $(3)$ a symbol, which is related to its referent only by convention $($as with words or traffic signals$)$