"Classic, or Classic Environment, was a hardware and software abstraction layer in Mac OS X that allowed applications compatible with Mac OS 9 to run on the Mac OS X operating system. The name "Classic" is also sometimes used by software vendors to refer to the application programming interface available to "classic" applications, to differentiate between programming for Mac OS X and the previous version of the Mac OS.
Classic is not supported in Mac OS X v10.5 Leopard, or on Intel Macs."
Classic Environment - Wikipedia, the free encyclopedia