Definition of folios:
(n) :
A page of a book, that is, one side of a leaf of a book.
(n) :
A leaf of a book or manuscript.
(n) :
(by extension, printing) A page number. The even folios are on the left-hand pages and the odd folios on the right-hand pages.
(n) :
(by extension, printing) A book made of sheets of paper each folded in half (two leaves or four pages to the sheet); hence, a book of the largest kind, exceeding 30 centimetres in height.
(n) :
(by extension, printing) A sheet of paper folded in half.
(n) :
(accounting) A page in an account book; sometimes, two opposite pages bearing the same serial number.