Definition of octennial in US English:

octennial

adjective

  • 1Recurring every eight years.

    • ‘But this explanation appears to be inconsistent with the octennial period of the festival.’
    1. 1.1 Lasting for or relating to a period of eight years.
      • ‘The Bill was, it is true, changed from a septennial to an octennial one.’

Origin

Mid 17th century: from late Latin octennium ‘period of eight years’ + -al.

Pronunciation

octennial

/äkˈtenēəl/