Short for Octavo: the most common size for current hardcover books. The sheet is folded three times into eight leaves (16 pages) up to 9.75" (~ 23 cm) tall.
When buying a used book online - Book Description: First Edition, First Printing. 8vo. Original red cloth-backed boards, titles to spine in gold and red. With the dustjacket. A fine copy in the dustjacket with a very few minute nicks.