Actually, if we're nit-picking, it means "Personal Computer", but the colloquial meaning has shifted somewhat since the good old IBM times to first mean desktop computers (as opposed to laptops), and then to mean non-Apple computers (including laptops), which for most people means "a computer that runs Windows."
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.