1. The earliest stages or first traces of anything.
2. Extant copies of books produced in the earliest stages (before 1501) of printing from movable type.
Somewhere in the incunabulum of time the New Year lay awaiting birth. Outside, in the dark, and with none to call it friend, the old year was fading into death. -- Edwin Markham, The real America in romance, Volume 12
Every sane soul is an incunabulum -- David Graham, The grammar of philosophy: a study of scientific method
Incunabulum derives, in the early printing press industry, from the Latin incuna , "to place a baby in a cradle."