PC is a computer based on IBM PC compatible standard, so usually x86 processor architecture with compatible with it components.
The term is so common that in practical language people started to use it as a replacement of the "desktop PC" or overall anything that is not pocketable or Apple.
But I guess with such question from OP it does not matter, as computers at the edge of the definition (like x86 Android tablets) are in a fraction of percent and won't matter in "what's the most popular".