ogdoad (OG-doh-ad) - n., a group of eight.
Sometimes specifically the eight deities worshiped in as a pantheon in Hermopolis in Old Kingdom Egypt, but in general, any octad. As to why we have two words for the same thing from the same Greek root, well, as I understand it, oktÅ is ancient Greek for the cardinal number eight, which changes to ogdoos in ordinal form eighth, and it's the odinal that was used to form ogdoas, eight-fold, while the English coinage (following English grammar?) uses the cardinal to get octad. There's a little bit of guesswork in that, tho', as Greek grammar is not my strong suit.
---L.
Sometimes specifically the eight deities worshiped in as a pantheon in Hermopolis in Old Kingdom Egypt, but in general, any octad. As to why we have two words for the same thing from the same Greek root, well, as I understand it, oktÅ is ancient Greek for the cardinal number eight, which changes to ogdoos in ordinal form eighth, and it's the odinal that was used to form ogdoas, eight-fold, while the English coinage (following English grammar?) uses the cardinal to get octad. There's a little bit of guesswork in that, tho', as Greek grammar is not my strong suit.
---L.