Vagueness is a property of concepts, terms, or representations characterized by a lack of sharp boundaries, making it genuinely indeterminate whether certain borderline cases fall within or outside their extension. As a research concept, it investigates the nature, structure, formal representation, and cognitive processing of such indeterminate boundaries and associated phenomena like the sorites paradox. Its significance lies in understanding fundamental characteristics and limits of language, logic, and knowledge representation, and in developing methods for handling imprecision in formal systems and communication across various domains.