Octodecimo is a term used in bookmaking that refers to a book size. It's a book format that measures approximately 4.25 inches by 6.875 inches when closed. Generally, books in this format are designed to be more portable than other formats, so they're often useful for travel or commuting. If you're looking for synonyms for the term Octodecimo, there aren't many common ones, but there are certain phrases you can use instead. For example, small-format book, pocket-size book, or miniature book are all terms that can be used to describe a book in the Octodecimo format. Ultimately, they all refer to books that are small, compact, and easy to carry.