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.

From the Guide to Literary Terms by Jack Lynch.
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Note: This guide is still in the early stages of development.
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