..

If somebody came up and asked me that question, I'd probably say something along the lines of 'A body of pages bound together at a spine with some sort of cover'- possibly not the most eloquent of descriptions so I'll consult the dictionary instead and see what it says:

"a written work or composition that has been published"
"physical objects consisting of a number of pages bound together"
"A collection of sheets of paper bound together to hinge at one edge, containing printed or written material, pictures, etc."
"a set of written, printed, or blank sheets bound together into a volume"
"a set of written sheets of skin or paper or tablets of wood or ivory"

Those are a few among many of the definitions given by various sources that I've come across, some are very specific about what they should contain i.e. paper, written work, pictures etc. whereas others are a lot more ambiguos, 'physical objects'? The last entry is interesting as it makes me think of ancient writings, i.e. 'books' of the bible - most, if not all were not written in traditional 'book' form, and instead were written on scrolls mainly on papyrus - yet we call them books.