Carbon

Carbon is one of Apple’s C-based application programming interfaces (APIs) for macOS (formerly Mac OS X), the operating system that powers Macintosh computers. Carbon provided a good degree of backward compatibility for programs that ran on Mac OS 8 and 9. Developers could use the Carbon APIs to port their “classic” Mac software to the Mac OS X platform with little effort, compared to porting the app to the entirely different Cocoa system, which originated in OPENSTEP.

Carbon was an important part of Apple's strategy for bringing Mac OS X to market, offering a path for quick porting of existing software applications, as well as a means of shipping applications that would run on either Mac OS X or the classic Mac OS. As the market has increasingly moved to the Cocoa-based frameworks, especially after the release of iOS, the need for a porting library was diluted. Apple did not create a 64-bit version of Carbon while updating their other frameworks in the 2007 time-frame, and eventually deprecated the entire API in OS X 10.8 Mountain Lion, which was released on July 24, 2012. Carbon was officially discontinued and removed entirely with the release of macOS 10.15 Catalina.

Source https://en.wikipedia.org/wiki/Carbon_%28API%29