Swift is Apple's modern, type-safe programming language for iOS, macOS, watchOS, and tvOS development, introduced in 2014 to replace Objective-C. Designed with protocol-oriented programming at its core, Swift combines the performance of compiled languages with the expressiveness of scripting languages while prioritizing safety through features like optionals and strong typing. A key mental model: Swift encourages value types (structs) over reference types (classes) for better performance and predictability — think "copy-by-default" rather than "share-by-reference," which fundamentally shapes how you architect Swift applications.