Definition - What does iOS SDK mean?
The iOS SDK was formerly known as the iPhone SDK.
Techopedia explains iOS SDK
In general, a software development kit (SDK) consists of tools used for developing applications for a particular platform. An SDK includes an application programming interface (API), which serves as a link between software applications and the platform they run on. APIs can be built in many ways and include helpful programming libraries and other tools.
Software development kits may be licensed by the software provider. They are typically made to assist with platform-specific development that goes on outside of a given company, for example, when individual developers submit applications or tools to be used with a particular platform.
The iOS SDK dates back to 2007 when Steve Jobs announced the development of this resource in 2008. The iPhone SDK provides tools for Apple's touchscreen interface and for its proprietary iOS operating system, which is used to run Apple's iPhones as well as Apple's other mobile devices such as the iPad.