spine definition

1. a small sharp-pointed tip resembling a spike on a stem or leaf
2. the part of a book's cover that encloses the inner side of the book's pages and that faces outward when the book is shelved
• examples: the title and author were printed on the spine of the book
3. a sharp rigid animal process or appendage; as a porcupine quill or a ridge on a bone or a ray of a fish fin