The planets, without Pluto

• Phases of the moon, say
• Two quartets
• Oxygen atom's protons, e.g.
• The planets, e.g
• Quarterfinals group, perhaps
• Santa's reindeer team, sans Rudolph
• Rowing team, e.g
• The cardinal number that is the sum of seven and one

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Octad \Oc"tad\, n. [Gr. ?, ?, the number eight.] (Chem.) An atom or radical which has a valence of eight, or is octavalent. A group of eight things.