I know I've seen it here and just want to know again what it is before I post a WL book.  There is a crease in the spine and it favors a couple pages but nothing is loose, no damaged pages-it's just been read several times (by me).I think I saw a link for a picture of what cleaving looks like.  But when I do an internet search I just get books that use the word "cleave" in the text somewhere.

I believe (and I could be wrong) that cleaving is when the glue has come unstuck and the pages actually fall out.

I thought it was where the spine was cracked and you could see through the page binding to the other side, no pages necessarily loose.  There was a thread with a picture but I can't remember for sure what it looked like

From what I understand it when the book is essentially more like 2 smaller books.  i.e. pages 1-150 are kind of in once big chunk and pages 151 - 300 are in another.  But, the chunks are still adhering to the spine and there is no gap between pages 150 and 151.  

Cleaving is to part or split, esp. along a natural line of division.  You can tell if your book has cleaving if you open it with the spine held and it naturally falls to the same page every time.  The natural line of divison would be between these pages.

The help center doesn't actully use the term "cleaved" in its conditions any more.  I think too many people were confused, because they didn't know what it means.  Here's what the help center actually says now:

Binding:

• Must be intact, with no separation on the inside or outside of the book
• If the book tends to open at a particular page, this is OKAY
• Small amount of fraying or denting at top or bottom is OKAY

So, the book falling open to one page is okay, but separation like Lita & Leslie explained is not okay.

Mary, your book sounds fine to post.

Ok then this book is postable.  It is in great condition except it tends to open to the same page. 

If you mean a creasing on the spine on the outside, that is totally postable! Just because it favors a couple pages, if you look at the pages and you don't see them trying to separate in spots, they are completely fine, only if there is some signs of separation is there any question, and then's when you get into degrees of opinions, but "favoring" is not an issue ;)