Our directory recently passed 4,000 APIs, each one different than almost every other one. There is a single defining factor of all 4,000: in some way, they're available for any developer to use. They're public. There is a virtual ocean below our directory of APIs that are currently private. These APIs drive mobile apps, connect strategic partnerships and exist within organizations large and small to facilitate data sharing.
Some of the APIs in our directory look like cousins of the private API. Their documentation is only available by request, or access is only offered to approved partners. And increasingly, there is a paid barrier to many we list. In some cases, the entire business is an API or collection of APIs.