A signature is a single section in a book with a sewn binding, either hardcover or paperback. It is a single piece of paper that may or may not have printing on it, folded and then trimmed at the edges, and sewn together, with other signatures, form the book block. A signature is generally 8, 16, or 32 pages, though it can be more or less.
One may identify a book with a sewn binding by noting that it is comprised of separate signatures, sewn together at the spine, rather than a mass of pages, glued together, and by the stitching between the center two pages of the signature, at the fold.
In most of the books printed before the 20th c., and some printed more recently, one can identify the first page of the signature by a letter at the bottom center of the first page of the signature. The signatures are lettered in chronological order, starting with A and going to Z, then from AA to ZZ, etc. This was done so that the bookbinder
would know the proper order to sew the signatures together in.