The 'Greater Good'. The Tau's driving force supposedly. This feeds directly into some of the ideals of Communism and Socialism, however, the 'Tau Empire' functions as an Imperialist state, despite not being an actual empire. Interestingly enough, there is also the occasional example of Communism.
Any nation - be it false or not - can call themselves whatever they wish, however, actions speak louder then words.
Communism is the idea of a society functioning for 'The Greater Good' of everyone without a large centralized government in control - the original idea anyway, i.e. Marxism. No unnecessary over production of goods, most everyone has a trade skill, etc, etc.
Concerning the Tau however, this really isn't a matter of government but more along the lines of religion.
They -function- as an Imperial state by definition, however, it is their belief that provides the drive behind their actions. All assimilated species are taught about the Greater Good - each 'invasion' is for the Greater Good. In some of the books, the Tau use trade and limited medical/technological support on Imperial worlds too small to matter to slowly spread and turn humans to the Greater Good, almost like Missionaries.
This is their belief structure. In a way, it's like a watered downed version of Christianity which has done the same thing over the years - subverting indigenous populations using aid, and in less savory scenarios, war.
The Tau have always been interesting to me because there isn't really one thing you could call them. In fact, in the real world - or I should say, with humans - their whole system just wouldn't work. It's nearly a perfect form of government - each caste needing the other, working their separate roles in unison, driven by this belief of doing the Greater Good for all intelligent forms of life. This is accomplished with the creation of the Ethereal caste, who rallied Tau and divided them into their positions, giving them a drive. Before that, the Tau were small groups that fought amongst themselves constantly.
# the domain ruled by an emperor or empress; the region over which imperial dominion is exercised # a group of countries under a single authority; "the British empire" ... Septs... # a monarchy with an emperor as head of state # conglomerate: a group of diverse companies under common ownership and run as a single organization
yeh tau would be a conglomerate empire
id call the Tau a Coalition, not an empire or imperialism