Other forms: folios
A folio is a large book made of sheets folded once. The First Folio, a folio edition, bound in folio.
The word names the size rather than the contents: one fold gives two leaves and the largest ordinary book. The First Folio of 1623 preserved eighteen Shakespeare plays that exist nowhere else.
1nounfolioA large book made of sheets folded once.
“The library's rarest treasure is a huge leather-bound folio of medieval maps.”
folio is a book.
Latin folium means a leaf, and folio is the ablative, as used in the phrase in folio.
Proto-Indo-European
*bʰleh₃-
Latin
foliō
Medieval Latin
foliō
Middle English
folio — leaf of a book
English, today
folio
From Middle English folio (“leaf of a book”), borrowed from Medieval Latin foliō, Latin foliō, Latin foliō, the ablative singular form of Latin folium (“leaf or sheet of paper”), Latin folium (“leaf of a plant”), ultimately from Proto-Indo-European bʰleh₃- (“bloom, flower”). a doublet of foil and folium, and distantly related to a doublet of phyllo and phyllon. Senses 1, 2, 3.1, 5, and 6 relating to a leaf or page are derived from Medieval Latin foliō in references; sense 5 (“page in an account book”) may be derived from Italian foglio (“rectangular sheet of paper”), from Latin folium. Senses 3.2 and 3.3 relating to a paper size are from Italian in foglio or its etymon Latin in foliō.
Etymology from Wiktionary, CC BY-SA