Glossary Term Definition
A page number, or a sheet of paper folded once to make two leaves (four pages).
Definition
A page number. Also, a sheet of paper folded once to make two leaves (four pages). In bibliographic usage, a folio is a book made of sheets folded once, producing pages roughly 38 × 25 cm or larger.
Source
No access — Robert Bringhurst — p. 324:
A page number, or a sheet of paper folded once to make two leaves.