a sharp rigid animal process or appendage; as a porcupine quill or a ridge on a bone or a ray of a fish fin
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; "the title and author were printed on the spine of the book"
a small sharp-pointed tip resembling a spike on a stem or leaf
a certain degree of unwillingness; "a reluctance to commit himself"; "his hesitancy revealed his basic indisposition"; "after some hesitation he agreed"
(physics) opposition to magnetic flux (analogous to electric resistance)