Relatum is a Latin word for anything that relates to something else.
For Ferdinand de Saussure, the sign is in its essence the relation between the signifier and the signified, and thus the signifier and the signifier are Relatum in the sign relation.
As semiotics seeks to be an all-inclusive theory of signs, it is forced to rise to ever-higher levels of abstraction and universality.
For example, to achieve the concept of the sign in general, it abstracts from the differences in words between spoken and written language and even goes further, it abstracts from the differences between verbal and nonverbal signs.
To frame these abstractions, semiotics is forced to use extremely general terms such as relatum and mediation. It needs words that can, without losing their meaning, cover a wide range of diverse phenomena.
The need to construct highly abstract models of the sign relation or function is balanced by the opposite need to develop a finely nuanced typology (or classification) of the signs. While models are designed to represent a single set of character types, typologies are constructed to illuminate the set available in the semiotic domain.
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