means A software development kit—a pre-built collection of tools, libraries, documentation, and code samples that lets developers build applications on top of a platform or service without starting from zero.

from Emerged in the 1980s as software got complicated enough that companies realized giving developers ready-made building blocks was cheaper than explaining everything from scratch. IBM, Microsoft, and Apple all shipped SDKs to encourage third-party apps. The acronym stuck because it beat saying 'here's a box of code we made for you.'