hollow back binding
OAB Definition
Bindings in which the spine of the text block is not affixed to the spine of the cover material, but rather, a hollow is used to join the two, leaving an empty space in the spine area between the text block and the covering material.
ABT Definition
Bindings in which the spine of the text block is not affixed to the spine of the cover material, leaving an empty space or hollow in the spine area between the text block and the covering material.