These people keep bringing up "mArKeT sHaRE"
EDIT: I wonder if Microsoft are making anti-Linux accounts to try and hamper Linux.
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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It would have been anything that implements Bios enough to boot MS-DOS, more or less.
But now that's not what anyone actually wants anymore since Windows, the thing people usually boot, wants UEFI instead. So I would say now it is probably anything that can run x86 code and boot Windows, even if it's from System76 and meant to run Linux.