League

A League is an alliance of Factions. The strongest House among the members of a League is considered a Great House and the ruler of that League.

Codex
"When a strong leading house seeks to increase its power, it could force a nearby colony to become its vassal by conquering that colony from a weaker faction and installing a new ruling house that swears fealty to its new master.

Another approach is for the weaker faction as a whole to become the vassal of the stronger one. This is frequently beneficial for both of the leading houses. The weaker leading house gets to retain most of its power and wealth, while gain the protection of the stronger one. On the other hand, the stronger house avoids a protracted and potentially bruising military campaign. Furthermore, there is a practical limit to how many vassals a leading house can effectively manage, and having a single leading house as a vassal is easier than managing the ruling house of each colony separately.

When a leading house has another leading house as a vassal, it is considered a great house, and the territory it controls is called a league.

When the power structure of a league has been stable for a long time, it becomes widely accepted that its dominant faction is the legitimate authority of the league, and that it should have mastery over its vassal factions."