Classic, or Classic Environment, was a hardware

Computer hardware

Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...

 and software abstraction layer

Abstraction layer

An abstraction layer is a way of hiding the implementation details of a particular set of functionality...

 in Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

 that allowed applications

Application software

Application software, also known as an application or an "app", is computer software designed to help the user to perform specific tasks. Examples include enterprise software, accounting software, office suites, graphics software and media players. Many application programs deal principally with...

 compatible with Mac OS 9

Mac OS 9

Mac OS 9 is the final major release of Apple's Mac OS before the launch of Mac OS X. Introduced on October 23, 1999, Apple positioned it as "The Best Internet Operating System Ever," highlighting Sherlock 2's Internet search capabilities, integration with Apple's free online services known as...

 to run on the Mac OS X operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

. The name "Classic" is also sometimes used by software vendors to refer to the application programming interface

Application programming interface

An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 available to "classic" applications, to differentiate between programming for Mac OS X and the previous version of the Mac OS. Classic is supported on PowerPC

PowerPC

PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...

-based Macs running versions of Mac OS X up to v10.4 Tiger

Mac OS X v10.4

Mac OS X v10.4 Tiger is the fifth major release of Mac OS X, Apple's desktop and server operating system for Macintosh computers. Tiger was released to the public on 29 April 2005 for US$129.95 as the successor to Mac OS X Panther , which had been released 18 months earlier...

, but is not supported on v10.5 Leopard

Mac OS X v10.5

Mac OS X Leopard is the sixth major release of Mac OS X, Apple's desktop and server operating system for Macintosh computers. Leopard was released on 26 October 2007 as the successor of Tiger , and is available in two variants: a desktop version suitable for personal computers, and a...

 and later, nor on Intel-based Macs running any version of Mac OS X.
Classic was a descendant of Rhapsody's "Blue Box" emulation layer, which served as a proof of concept

Proof of concept

A proof of concept or a proof of principle is a realization of a certain method or idea to demonstrate its feasibility, or a demonstration in principle, whose purpose is to verify that some concept or theory that has the potential of being used...

.