SDK
In plain terms
A bundle of tools and libraries for building on a platform.
Going deeper
A Software Development Kit provides the libraries, tools, docs, and samples developers need to build applications for a specific platform or service. It's a build-on-me starter pack.
Under the hood
Broader than an API library; may include compilers, debuggers, and emulators.
Examples
Android SDK, AWS SDK, iOS SDK, platform development, cloud integrations.
Think of it like...
Like a complete toolkit with parts and instructions for building on a platform.