Definition of "octennial" [oc•ten•ni•al]

• Occurring every eight years. (adjective)
• Lasting or taking eight years. (adjective)

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• "These 90 days he divided into 2 months of 22 and 2 of 23 days, ([2 × 22] + [2 × 23] = 90,) and introduced them alternately every second year for two octennial periods: every third octennial period, however, Numa intercalated only 66 days instead of 90 days, _i.e. _ he inserted 3 months of only 22 days each."
• "Now an octennial cycle is the shortest period at the end of which sun and moon really mark time together after overlapping, so to say, throughout the whole of the interval."
• "Without being unduly rash we may surmise that the tribute of seven youths and seven maidens whom the Athenians were bound to send to Minos every eight years had some connexion with the renewal of the king’s power for another octennial cycle."