"tablet pc" is a technical term for a certain size/shape/functionality of a PC. Another word (like handheld, palm, etc.) would probably distort the meaning
I think a "tablet" PC is a bit smaller than a "laptop" PC and has the additional feature of recognizing writing-like motions on its swivel-jointed screen, translating them into text files. It's probably good for people who don't like keyboards. I hope this explanation didn't make you sorry you asked the question! 👁 Smile :)