A sign's an entity which signifies another entity. A natural sign's an entity which bears a causal relation to the signified entity, as thunder's a sign of storm. A conventional sign signifies by agreement, as a full stop signifies the end of a sentence. (Contrast a symbol which stands for another thing, as a flag may be a symbol of a nation) The way in which a sign signifies's a topic in philosophy of language, see also Meaning (linguistic). Any given signifier or symbol's dependent upon that… (More on Signification)