[ 'kwɔ:təʊ ]
(abbrev.: 4to )
■ noun ( plural ~s ) a page or paper size resulting from folding a sheet into four leaves, typically 10 inches × 8 inches (254 × 203 mm).
↘a book of this size.
Origin
C16: from L. (in) ~ '(in) the fourth (of a sheet)', ablative of quartus 'fourth'.