Here’s a word I’ve never heard before. I wonder if I can get this to be widely used.
ogdoad \OG-doh-ad\ noun
1. the number eight.
2. a group of eight.
The monad, [that is,] the one tittle, is therefore, he says, also a decade. For by the actual power of this one tittle, are produced duad, and triad, and tetrad, and pentad, and hexad, and heptad, and ogdoad, and ennead, up to ten. For these numbers, he says, are capable of many divisions, and they reside in that simple and uncompounded single tittle of the iota.
— Hippolytus (170–235), The Refutation of All Heresies, translated by J. H. MacMahon, 1851
Ogdoad came to English from the Late Latin in the early 1600s and ultimately derives from the Greek ógdoos meaning “eighth.”