The area along the edges of the spine along which the boards and/or cover hinge. It is sometimes useful to distinguish between the outer joint, which describes the cover on the outside of the boards/cover, and the inner joint, which can be used to describe those parts of the endleaves and spine linings which cross the joint on the inside of the board/cover.
Roberts, Matt, Don Etherington, and Margaret R. Brown. 1982. Bookbinding and the conservation of books: a dictionary of descriptive terminology. Washington: Library of Congress.