An incunable, or sometimes incunabulum (plural incunables or incunabula, respectively) is a book, pamphlet, or broadside (such as the Almanach cracoviense ad annum 1474, Wikipedia page ) that was printed—not handwritten—before the year 1501 in Europe. "Incunable" is the anglicised singular form of "incunabula", Latin for "swaddling clothes" or "cradle" which can refer to "the earliest stages or first traces in the development of anything." A former term for "incunable" is "fifteener," referring to the fifteenth century.
The first recorded use of incunabula as a printing term is in a Latin pamphlet by Bernhard von Mallinckrodt, De ortu et progressu artis typographicae ("Of the rise and progress of the typographic art", Cologne, 1639), which includes the phrase prima typographicae incunabula, "the first infancy of printing", a term to which he arbitrarily set an end, 1500, which still stands as a convention. The term came to denote the printed books themselves in the late seventeenth century.
More information on the Wikipedia page