Learner's definition of SPINE
1 [count] : the row of connected bones down the middle of the back : backbone 2 [count] : a sharp, pointed part on an animal or plant 3 [noncount], informal : courage or strength of character 4 [count] : the part of a book to which the pages are attached
◊ A book's spine often has the title and author's name printed on it.
up/down your spine
◊ If something sends a chill/shiver up/down your spine or if a chill/shiver runs up/down your spine, you become frightened, thrilled, etc.
Her performance of the national anthem sent a chill up my spine. [=her performance was thrilling and emotional]
The very thought of what those people did sends a shiver down my spine. [=scares me]
A (cold) shiver ran down my spine when I heard his voice.