How did Charles Sanders Peirce classify signs?

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Semantics German philosopher of mathematics, logic, and language Gottlob Frege.

The American philosopher Charles Sanders Peirce classified signs based on the relationship between the sign and its referent. He defined a sign as "something which stands to somebody for something" and categorized them into three primary types:

Icon: A sign that bears a resemblance to its referent, such as a road sign depicting falling rocks.1
Index: A sign that maintains a direct association with its referent, such as smoke serving as a sign of fire.1
Symbol: A sign that relates to its referent solely through convention, such as words or traffic signals.1

Peirce further emphasized that the meaning of a sign is never fixed or definite, as it must be continuously qualified. This work, rooted in pragmatism and logic, remains a foundational pillar of semiotics, the study of signs and sign-using behavior.1