It's a difficult thing to define, because once you start defining it you can second-guess yourself, resulting in a very broad definition which encompasses everything from your pacemaker to your toothbrush.
Computer: Electronic shit with a processor.
In my opinion the term PC means something other than Personal Computer. That term came from the day where it was pro that you could fit a computer in your house. So a definition to suit the times:
PC: Keyboard and mouse for user input, unless special accessibility requirements are present. Must have a minimum available output resolution of 1280x1024. Consists of a processor, a motherboard, a graphics adapter, a data store, ram, audio output. Must run an operating system capable of networking, and interfacing with devices such as printer and usb storage.
That's just me eating my breakfast and making up a definition, so please refine it. I'm thinking along the lines of "If I bought a PC, what would I assume it did?"