OK, so I find my old comment, and obviously at one point in my life, I must have known what this word meant. So I blithely click on all the cute little squares to remind myself... and none of them list the dang word! So I'm pasting the definition here so I won't get annoyed when another seven months go by and I forget what it means again.
Noun. A manuscript, parchment, or book having writing on only one side of the leaves.