1.nounA size of paper (7.5"-10" x 10"-12.5" or 190-254 x 254-312 mm). Formed by folding and cutting one of several standard sizes of paper (15"-20" x 20"-25" or 381-508 x 508-635 mm) twice to form 4 leaves (eight sides).
2.nounA book size, corresponding to the paper size.