The programming language predominantly used for iOS app development is Swift. Introduced by Apple in 2014, Swift offers a modern syntax and enhances code readability while integrating safety features that help prevent common programming errors. Swift is designed to be compatible with existing Objective-C code, allowing developers to transition and utilize previous codebases efficiently. Its performance, ease of use, and active community support make Swift the preferred choice for developers creating applications within Apple's ecosystem, including iPhone and iPad apps.