Heptarchy is the name historians apply to the period of English history after the Anglo-Saxons conquered England, but before the Vikings started their predations in that part of Britain. The beginning and end dates of this period are usually given as AD 500 to 850. Use of this term dates back as early as Henry of Huntingdon, an English historian of the twelfth century, and has been common since the sixteenth century.
This term, heptarchy, is Greek for "seven rulers", referring to the fact that between the two years mentioned above it was thought that England was divided into seven kingdoms: Northumbria, Mercia, East Anglia, Essex, Kent, Sussex, and Wessex. However more careful study has shown that on one hand some of these kingdoms -- notably Essex and Sussex -- never achieved the same rank of power in England at any time as did the rest, while after an early period of renown Kent was considered a minor kingdom under the sway of one of the other kingdoms, much as Essex and Sussex were. Further, research has shown that a number of political divisions played a far more important role than were earlier thought; these kingdoms include Lindsey, the Hwicce, Magonsaete, Surrey, the Wihtware or inhabitants of the Isle of Wight, the Middle Angles, and the Gewissae.
In short, this term has been considered unsatisfactory since the early twentieth century, and many historians have ceased using it, feeling it does not adequate describe the period it was meant to apply to. However, many historians and scholars still use it as a convenient shorthand for the period.