Carbon
Carbon was a transitional technology that supported developers of classic Mac OS apps during the transition to Mac OS X.
When Apple unveiled their Rhapsody and Mac OS X Server 1.x strategy, the plan for running Mac OS 9 apps was to run them in “the blue box,” a virtual Mac OS 9 environment. However, many developers voiced opposition to this strategy.
In response, Apple shifted to a new strategy. Carbon was a library that implemented the majority of classic Mac OS API calls in terms of the new Cocoa APIs. This meant that classic app developers could make minor changes to their app to allow it to run under Carbon and get many of the benefits of Mac OS X.
I found a podcast where Sean Parent gave some interesting history about Carbon: