Definition of Folio
A leaf of a book or manuscript. A page number. In the ancient lawbooks it was the custom to number the leaves, instead of the pages; hence a folio would include both sides of the leaf, or two pages. The references to these books are made by the number of the folio, the letters "a” and b” being added to show which of the two pages is intended.
A large size of book, the page being obtained by folding the sheet of paper once only in the binding. Many of the ancient lawbooks are folios.
When used in connection with legal documents, it formerly meant a certain number of words varying from 72 to 100, but generally in the United States it consisted of 100. Such was used as a unit for measuring the text length of the legal instrument.
Placing a serial number on each leaf or page of printed matter
That's the definition of Folio in Black's Law Dictionary 6th Edition. Courtesy of Cekhukum.com.