Definiteness is a semantic-pragmatic notion that is closely associated with the use of the definite article (or determiner) in languages like English, Hungarian, Hebrew, and Lakhota. The definite article can be used in different conditions: deictic, anaphoric, unique, and certain indirect uses, often also called “bridging uses.” Accordingly, there are different semantic theories of definiteness, such as the salience theory, the familiarity or identifiability theory, and the uniqueness or inclusiveness theory. Definite expressions cover personal pronouns, proper names, demonstratives, definite noun phrases, and universally quantified expressions. Noun phrases with the definite article, known as “definite descriptions,” are a key issue in semantics and analytic philosophy with respect to the interaction of reference and description in identifying an object. The research and analysis of definiteness is of great importance not only for the linguistic structure of languages but also for our understanding of reference and referring in philosophy, cognitive science, computational linguistics, and communication science.