Other forms: incunabula
Incunabulum refers to a book that was printed in the first few decades when the printing press was introduced. That means printed before 1501.
The printing press with the movable metal type was invented around 1440 and made a huge difference to the production of books, which previously had to be copied or printed by hand. Incunabulam refers to an actual book printed during around this time, while the processes were still being refined. 1501 is a bit of an arbitrary date though — there's no particular development that separates books printed in 1499 from books published in 1502.