Definition of: octad

(ok′tad) noun
1. A series of eight.
2. Chem. An atom, radical, or element that has a combining power of eight.
3. In ancient notation, a group of eight figures arranged similarly to successive powers of ten. [<L octas, -adis <Gk. oktas, -ados a group of eight <oktō eight]
—oc·tad′ic adjective