The Collaborative International Dictionary of English v.0.48:

ogdoad \og"do*ad\ ([o^]g"d[-o]*[a^]d), n. [Gr. 'ogdoa`s,
   'ogdoa`dos, from 'oktw` eight.]
   A thing made up of eight parts. --Milman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ogdoad
    n 1: the cardinal number that is the sum of seven and one [syn:
         eight, 8, VIII, eighter, eighter from Decatur,
         octad, ogdoad, octonary, octet]