Wiktionary:Word of the day/November 18

1. A leaf of a book or manuscript.
2. A page of a book, that is, one side of a leaf of a book.
3. (by extension, printing)
   1. A page number. The even folios are on the left-hand pages and the odd folios on the right-hand pages.
   2. A sheet of paper folded in half.
   3. 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.
4. A wrapper for loose papers.
5. (accounting) A page in an account book; sometimes, two opposite pages bearing the same serial number.
6. (law, dated) A leaf containing a certain number of words; hence, a certain number of words in a piece of writing, as in England, in law proceedings 72, and in chancery, 90; in New York, 100 words.

folio v

1. (transitive) To put a serial number on (a folio or page, or on all the folios or pages of a book); to foliate, to page.

The First Folio of the English playwright William Shakespeare’s plays, regarded as one of the most influential books ever published, was entered into the Stationers’ Register on this day (on 8 November according to the Julian calendar) in 1623.