octad
(ˈɒktæd)
[ad. L. octas, octad-, a. Gr. ὀκτάς, -άδα a group of eight: see -ad 1 a.]
1. A group or series of eight; spec. in ancient systems of arithmetical notation: A group or series of eight characters corresponding to successive powers of ten (analogous to the groups of six figures marking millions, billions, etc. now used).
| 1883 Sir E. C. Bayley Geneal. Mod. Numerals ii. in Jrnl. R. Asiat. Soc. XV. 48 [The Greeks] had, however, a system of ‘octads’ and ‘tetrads’ for expressing numbers of very high value. Ibid. 49 By collecting the alphabetical signs in groups of eight or ‘octads’, decimally arranged. |
2. Math. a. Mod. Geom. The set of eight intersections of three quadric surfaces.
| 1889 Cayley in Messenger XVIII. 149 The eight points of intersection of any three quadric surfaces are an octad. |
† b. pl. A system of eight imaginaries analogous to quaternions, also called octaves. Obs.
| 1845 J. T. Graves in Phil. Mag. XXVI. 315–20. 18.. Cayley Collected Papers I. 586. |
3. Chem. An element or radical that has the combining power of eight units, i.e. of eight atoms of hydrogen.
| 1877 Watts Fownes' Chem. I. 268 Ru[bidium] and Os[mium] form tetroxides (analogous to octochlorides), and may therefore be regarded as octads. |
Hence octadic (ɒkˈtædɪk) a., of or pertaining to an octad. octadic surface (Mod. Geom.), a quartic surface, eight of whose nodes form an octad.
| 1870 Cayley in Proc. Lond. Math. Soc. III. 20. |