It's short form for "IBM PC Compatible", something that was printed on a lot of floppy disks back in the early 1980s. It was used to differentiate between "Atari" (which was a computer), "Apple" (which wasn't a Macintosh), "C64", and "Tandy" in the USA, and certainly others elsewhere in the world.

The primary use of the term among (semi-)technical people has evolved to mean the defacto open standard personal computer platform that has a BIOS and uses the Intel 32 bit / AMD 64 bit instruction set.

The use of PC to literally mean "Personal Computer", a computer used by one person, is falling out of vogue. Apple obviously is running from the term, and as far as I can think the only other non-Windows platforms (Sun, for example) fancy themselves to be "Workstations" -- users and vendors alike would take umbrage at the suggestion that they're mere PCs, even though they're the same size, shape, come with a keyboard and mouse, and are often less powerful.