What Does Mac OS X Mean?
Mac OS X is a line of Apple operating systems (OS) included in every modern Macintosh (Mac) computer. It is the successor to the classic Mac OS 9. In March 2001, Apple released the first Mac OS X operating system to hit the market: Mac OS X 10.0 Cheetah. Mac OS X operating systems are based on UNIX.
Techopedia Explains Mac OS X
The origin of Mac OS X goes back to the NeXTSTEP OS, which was launched by NeXT - a company founded by Steve Jobs. Because its hardware was not as commercially successful as anticipated, focus shifted to its software. Like OS X, NeXTSTEP was an object oriented and multitasking OS. Jobs took the NeXTSTEP model with him when he returned to Apple, which eventually acquired NeXT.
As recently as Mac OS X 6 Snow Leopard, NeXTSTEP icons were used in OS X. For example, when pressing command+shift+4 and the space bar, a NeXTSTEP camera icon appears.