The primary language used for iOS app development is Swift. Developed by Apple, Swift is designed to be powerful yet easy to learn, featuring modern programming constructs. It replaced Objective-C as the main programming language for iOS, allowing developers to create apps that are more efficient, safe, and expressive. Swift's interactive environment helps developers catch errors early, enhancing productivity. Additionally, Swift has gained support from various libraries and frameworks, further solidifying its role in the ecosystem of iOS application development.