1 : a book printed before 1501
2 : a work of art or of industry of an early period
The invention of the mechanized printing press in the 15th century revolutionized the way books were produced, dramatically increasing the number and variety of works to be published and distributed to awaiting readers. "Incunabulum" first appeared in English in the 19th century, referring retroactively to those books produced in the first decades of printing press technology, specifically those printed before the year 1501, a date that appears to have been determined only arbitrarily. Coming from Latin, "incunabulum" is singular of "incunabula," which translates literally to "swaddling clothes" or "bands holding the baby in a cradle." The "baby" in this case is likely a figurative one, referring to a book that was produced when the art of printing was still in its infancy.
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Source: Merriam Webster