"Deep structure" is a concept in linguistics introduced by Noam Chomsky. It refers to the underlying grammatical structure of a sentence that conveys its core meaning. This structure is abstract and not directly observable in spoken or written language, as it is transformed into various surface structures through rules of syntax. In transformational grammar, deep structure serves as the foundation from which different sentences can be generated. For example, the sentences "The cat chased the mouse" and "The mouse was chased by the cat" have different surface structures but share the same deep structure, reflecting the same underlying idea.