Incunabula has a couple of meanings. It can mean the earliest beginning of anything (you may notice a theme for January words!). However, incunable, incunabula and incunabulum refer to books or pamphlets printed before 1501.
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Page from Valerius Maximus, Facta et dicta memorabilia, printed in red and black by Peter Schöffer (Mainz, 1471).