Intension refers to the inherent properties or qualities that a word, phrase, or symbol connotes—the internal content that constitutes its formal definition. This contrasts with extension, which denotes the actual set of objects or instances a term applies to. For example, the intension of the term "bird" includes attributes like "feathered," "winged," and "capable of flight," whereas its extension encompasses all individual birds.

In semantics and the philosophy of language, understanding intension is crucial for grasping how language conveys meaning. It involves the relationship between signifiers (words or symbols) and what they signify—the concepts or ideas they evoke. Swiss linguist Ferdinand de Saussure highlighted this by distinguishing between the "signifier" (the form of a word) and the "signified" (the concept it represents).

Intensional definitions specify the necessary and sufficient conditions for a term's application, focusing on the inherent characteristics that define a concept. This is opposed to extensional definitions, which list all instances that fall under a term. For instance, an intensional definition of "triangle" would be "a three-sided polygon," emphasizing its defining properties, while an extensional definition would list all specific triangles.

In logic and linguistics, intensional contexts are those where substituting expressions with identical extensions does not preserve truth value. For example, the statements "John believes that the morning star is bright" and "John believes that the evening star is bright" may differ in truth value, even though "morning star" and "evening star" both refer to Venus. This phenomenon underscores the importance of intension in understanding meaning beyond mere reference.

Grasping the distinction between intension and extension is essential for analyzing language, meaning, and reference, as it illuminates how terms can convey different nuances and implications beyond their direct referents.