Signature.

A signature is a printed sheet folded down into a section of leaves, gathered in order with the others to make the book block. Folding fixes the arithmetic, since a sheet folds into sets of 4, 8, 16, 32, 64 or 128 pages, so an offset book's page count gets pushed toward a multiple of the signature and short books are padded with blanks rather than run odd. Strictly the word names only the identifying mark printed on a section's first page to guide the binder, with section being the folded paper itself, but the trade stopped distinguishing the two long ago (Etherington and Roberts, Dictionary of Descriptive Terminology). Sheet and section are not interchangeable either, because one sheet may carry more than one section and a half sheet makes a section with fewer leaves than its neighbours (gathering). Print-on-demand meets the term mostly by its absence: KDP takes a paperback at any page count from 24 to 828 and charges by the page above 110 (KDP printing cost), so nothing pushes a page count toward a multiple of sixteen, and the saving an offset planner works for is not there to be worked for.