<computer> The line of computers manufactured by Apple Inc. "Mac" is not primarily a nickname or an abbreviation, but a brand name and trademark in its own right. Apple currently (2009) refer to the brand as any of "Mac", "iMac" or "Macintosh" (all registered trademarks).
The Mac was Apple's successor to the Lisa. The project was proposed by Jef Raskin some time before Steve Jobs's famous visit to Xerox PARC. Jobs tried to scuttle the Macintosh project and only joined it later because he wasn't trusted to manage the Lisa project.
The first Macintosh, introduced in January 1984, had a Motorola 68000 CPU, 128K of RAM, a small monochrome screen, and one built-in floppy disk drive with an external slot for one more, two serial ports and a four-voice sound generator. This was all housed in one small plastic case, including the screen. When more memory was available later in the year, a 512K Macintosh was nicknamed the "Fat Mac."
The Mac SE (March 1987) had up to four megabytes of RAM, an optional built-in 20 megabyte hard disk and one internal expansion slot for connecting a third-party device.
The Macintosh Operating System is now officially called "Mac OS". Mac OS X is the successor to Mac OS 9, although its technological parent is the NEXTSTEP OS from Next, Inc., founded by Steve Jobs after he left Apple the first time. OS X is based largely on the BSD UNIX system. The core of the OS X operating system is released as free source code under the project name Darwin.
If "Macintosh" were an acronym, some say it would stand for "Many Applications Crash, If Not, The Operating System Hangs". While this was true for pre Mac OS 9 systems, it is less true for Mac OS 9, and totally incorrect for Mac OS X, which has protected memory, so even if one application crashes, the system and other applications are unaffected.