The Collaborative International Dictionary of English v.0.48:

Heptarchy \Hep"tarch*y\, n. [Hepta- + -archy: cf. F.
   heptarchie.]
   A government by seven persons; also, a country under seven
   rulers.
   [1913 Webster]

   Note: The word is most commonly applied to England, when it
         was divided into seven kingdoms; as, the Saxon
         heptachy, which consisted of Kent, the South Saxons
         (Sussex), West Saxons (Wessex), East Saxons (Essex),
         the East Angles, Mercia, and Northumberland.
         [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

HEPTARCHY, Eng. law. The name of the kingdom or government established by
the Saxons, on their establishment in Britain so called because it was
composed of seven kingdoms, namely, Kent, Essex, Sussex, Wessex, East
Anglia, Mercia, and Northumberland.