What is a Primary Relationship to You?

I've been reading posted responses to Poly topics, and I've seen several mentions of Primary Partner(s). So I'm just sort of wondering, what is the definition of a Primary Relationship to others here?

In my overall experience in the Poly Community over my LifeTime, I have met many people who have different beliefs Re; this question. And I have met many who claim to have an egalitarian view and practice in their Relationship(s). And yet I have yet to meet one person who actually Lives an egalitarian Life IE No Hierarchal Order in their Relationship(s).

So I ask, What defines a Primary Relationship to you?

Respectfully~SidheWolf