A reign is the period during which a monarch rules.
The Reign of SCA Kings
In most SCA Kingdoms, the King and Queen reign for six months at a time. This means that King and Queen of Lochac and Ealdormere, for example, only have to travel the vast distances of their kingdoms for six months before their successors take over.
This is generally considered a kindness for the monarchs' vehicles, and for the monarchs themselves.