What's the real definition between an e-book and a whitepaper?

This is a great question. I agree with Noreen that white papers are usually research papers, or even thinly veiled solution briefs with some surrounding factual data. They are based more on fact, than on opinion. Or at least in substantiating opinions with facts and statistics and tend to take a more distant stance than an eBook - which is often based on the author's expertise, alone.

An eBook has several additional differences that occur to me off the top:

It's formatted to be read in digital format - on a screen in landscape vs. portrait orientation. The idea being to make it easy to read. Ever notice that white papers are still formatted in columns, requiring the reader to scroll endlessly up and down to read one page?

An eBook has more white space and graphics. Due to the screen reading, not every inch of space is taken up with text. It's, as Noreen says, more visually engaging and easily digestible - due in large part to white space and design.

An eBook is also often designed to be more scannable. Think about shorter attention spans and the ease of getting the gist through headers and bullets with a cursory read that then proves value and has readers slowing down to read the content.

But, finally, the biggest difference to I see is that eBooks are more conversational. They're written from a more casual view which helps people "get" the ideas and encourages them to share them with others.

Because of that, an eBook has the potential to be more "viral."

Just a few thoughts. I'm not saying a white paper is not an effective content asset. It definitely has it's place. But, think about the purpose of your content and the audience who will read it when your choice of format is between an eBook and a white paper.