The term "Personal Computer" or "PC" for short predates both Macintosh and IBM both. It was a general term used to describe any kind of home computer regardless of make or model, to differentiate them from the room-sized mainframes and such.
Somewhere along the line, probably around the time 8088-based IBM PC clones started showing up on the market, the term's usage shifted to include only IBM PCs "and compatible."
Technically speaking, if you really want to be anal, the term "PC" is still valid when referring to a home computer whether it be a PC-clone, Mac, or otherwise. It's just that very few people in today's society will know what you mean if you refer to your Mac as a "PC."