What is a broken spine? Are they allowed as postables?

My understanding of a broken spine was when the glue was pulling away from the cover or old and cracked fully in half.

But now I am thinking it means when a paper back has the little ridges on the spine from being open wide over and over thus making them not perfect when you look at them on a shelf.

I ask partly because some of mind have been opened wide as the words are printed without a big enough margine so I have to open it wide to read the words.

 I am a little lost!

My understanding of a broken spine was when the glue was pulling away from the cover or old and cracked fully in half.

Yup.  That's a broken spine, and it's not postable.  Check the book condition guidelines in the Help Center

http://www.paperbackswap.com/help/search.php?terms=book+condition

But now I am thinking it means when a paper back has the little ridges on the spine from being open wide over and over thus making them not perfect when you look at them on a shelf.

No, I don't think that's a broken spine.  Not pretty, but postable.