"From the hacker’s point of view, APIs provide a great way of standing on the shoulders of giants, as well as making a service more useful for themselves and others. And if you’re really lucky, those others might even pay for your services."

[1] API’s and Mashups for the Rest of Us - Gareth Rushgrove – Digital Web

"Building an application with no APIs, says Josh Walker, an analyst at Forrester Research Inc. in Cambridge, Mass., ‘is basically like building a house with no doors. The API for all computing purposes is how you open the blinds and the doors and exchange information.’ APIs also exist between applications."

[2] QuickStudy: Application Programming Interface (API) - David Orenstein - Computerworld
 

"A language and message format used by an application program to communicate with the operating system or some other control program such as a database management system (DBMS) or communications protocol. APIs are implemented by writing function calls in the program, which provide the linkage to the required subroutine for execution."

[3] Definition of API – PC Magazine

"BOSS allows developers to write programs that utilize the Yahoo search index though an application programming interface. Yahoo has had search APIs, but with BOSS many of the usage restrictions are gone. This makes it possible to use Yahoo’s platform for a real business rather than hobby."

[4] Yahoo Wants You To Build Your Own Search Service – Information Week

" The MySpace platform is different from Facebook’s platform in a key way. Programmers will access MySpace’s platform through an open-source API (application programming interface) from OpenSocial, which Google, MySpace and other vendors created last year to standardize the way applications are created on social networking sites."

[5] MySpace Launches Developer Platform with OpenSocial – Clint Boulton