A Region is a term in use in some Kingdoms, notably the East, to describe an administrative sub-unit that has no recognition at the Corporate level. (Compare/contrast with Principality and Crown Principality.)
In the Kingdom of the Middle (and it's associated Kingdoms) it is a now mostly-defunct term to describe a trend towards clusters of baronies allying based on location. As the Midrealm once a covered a huge area, it was somewhat inevitable that SCAdians began identifying with others in the area, more than with the distant, over-arching kingdom. Such self-declared regions included Ealdormere and Northshield, among others. Calontir, as The Great Western Steppes, was the first, and folk there coined the term, available since it had no official meaning in SCA or Midrealm laws.
The Regionalism Debate
The growth of "regionalism" in the late 1980s, particularly in Canadian territories of the Midrealm, prompted a move towards making these regions offical, including appointing their own Regional Champions. It was generally regarded as a first step towards becoming a Principality, with the logical end of becoming an independant Kingdom one day.
It was a request from the baronies in what would become Ealdormere to establish an "Ealdormere Champion" that prompted the Ban on Regions, an attempt to halt "regional" growth and keep the Midrealm a single political and military force.
Many of the "regions" of the original controversy are now Principalities or Kingdoms.