Reading list: MacOS X
Mac OS X (with X representing Roman numeral ten) is an operating system for Apple’s Macintosh computers. In addition to many distinct end user functionalities, it supports multiple development technologies including UNIX, Java, the proprietary Cocoa and Carbon runtime environments, and a host of open source, Web, scripting, and database applications. Released in 2001, Mac OS X replaced its predecessor, “Classic” Mac OS version 9, effectively introducing a new nomenclature. The following timeline of updates used the same “Mac OS X” prefix:

Cheetah (v10.0) – March 2001
Puma (v10.1) – September 2001
Jaguar (v10.2) – August 2002
Panther (v10.3) – October 2003
Tiger (v10.4) – April 2005
Leopard (v10.5) – October 2007
Snow Leopard (v10.6) – June 2009
Lion (v10.7) – July 2011
Mountain Lion (v10.8) – July 2012
Mavericks (v10.9) – October 2013
Yosemite (v10.10) – October 2014
El Capitan (v10.11) – September 2015
With Sierra in 2016, Apple began using an even shorter naming convention, simply “macOS”:

Sierra (v10.12) – September 2016
High Sierra (v10.13) – September 2017
Mojave (v10.14) – September 2018
Catalina (v10.15) – October 2019
Big Sur (v10.16) – June 2020