The mother of all APIs in the Notes world!
Lotus decided to expose many of the real functions that the Lotus Notes client and the Domino server uses themselves. Refer to the discussion about Direct Exposure vs. bi-product delivery of APIs.
The basis for all API development with Lotus starts with the Notes C API. Several functions was exposed already from the release 2.0 of Notes, back in 1991. Since Lotus Notes exposes it's own functions, we can in a way say that Lotus Notes client and server itself is built with the Notes C API. Over the years the API has been richer and richer in functionality and the number of exposed functions and structures are steadily growing.