What do you call a wooden stick which you use to play cricket with? A bat. What do you call that same wooden stick if you use it to hit someone with? A weapon.
How we define something depends on how we use that thing. A bed is something you sleep on. A chair is something you sit on. A personal computer is something you do personal computing on.
So the question isn't about input method, size, power or anything like that. An iPad is a PC only if it is used as a PC is used. So what do most people use their PCs for? Email, web browsing, playing games, watching movies or TV shows, listening to music, shopping online, looking at photographs and social networking.
These things are what most people use their PCs for. I think we can agree on that. Does this list also apply to an iPad?
If so then the iPad is a PC by any reasonable definition.