Source : Webster's Revised Unabridged Dictionary (1913)

Septennial \Sep*ten"ni*al\, a. [L. septennium a period of seven
   years; septem seven + annus year. See {Seven}, and {Annual}.]
   1. Lasting or continuing seven years; as, septennial
      parliaments.

   2. Happening or returning once in every seven years; as,
      septennial elections in England.