An incunabulum — the plural is incunabula -- is a very early printed book, one produced before 1501. (Printing with movable type began in Europe around 1450-ish, so an incunabulum was produced in the first half-century or so of printing.)
Although the Latin term is more common, you might also hear it Englished as incunable.
Note: This guide is still in the early stages of development.
Three question marks mean I have to write more on the subject. Bear with me.