To upgrade from Mac OS X 10.4 (Tiger) to 10.5 (Leopard) you will need to buy a boxed version of Mac OS X 10.5.
If you have a Mac with an Intel processor you can go straight to Mac OS X 10.6 (Snow Leopard) which is a cheaper upgrade option.
Between 19, Apple sold a separate series of operating systems called Mac OS X Server.
The "X" in Mac OS X and OS X is the Roman numeral for the number 10 and is pronounced as such.
The X was a prominent part of the operating system's brand identity and marketing in its early years, but gradually receded in prominence since the release of Snow Leopard in 2009.
The heritage of what would become mac OS had originated at Ne XT, a company founded by Steve Jobs following his departure from Apple in 1985.
There, the Unix-like Ne XTSTEP operating system was developed, and then launched in 1989.