Ogdoad (ὀγδοάς) is an ancient Greek word for “eight.” The ancient Pythagoreans considered the numbers 1-10 each to embody certain qualitative aspects (in addition to their quantitative function). Speculation on the qualities of the first ten numbers is sometimes called “arithmology” by modern scholars, in order to distinguish it from from the modern sense of “arithmetic.”
On the Ogdoad (περὶ ὀγδοάδος):
Eight is the first cubic number (κύβος) and can thus be understood to be the dyad deployed in three dimensions (2 x 2 x 2). Noting the presence of eight nipples (θηλαί) in many species that nurse their young, the author of Theol. Arith. 74.5-10 continues: “Hence they used to call the ogdoad ‘mother’ (μήτηρ), perhaps referring to what has already been said (i.e., that even number is female), but perhaps, since Rhea (Ῥέα) is the mother of the gods, because although the dyad was shown to belong to Rhea seminally (σπερματικῶς), the ogdoad does in extension (ἐπέκτασις). And some think that the very word ‘ogdoad’ (ὀγδοάς) was coined to resemble ‘ekdyad’ (ἐκδυάς) – that is, the one which is generated ‘out of the dyad’ (ἐκ δυάδος) when it is cubed.”