I'm not actually sure how I personally define it.
I share everything with my wife except for sex. She's my best friend, my emotional companion, someone who shares my life goals. We're financially bonded, and this seems especially relevant to this question given it's nature.
I LIVE with my wife, I don't like with anybody else.
I suppose that loose defintion is what I mean by "primary". I don't have fuck buddies. I love everyone that I love "the same". My love for one doesn't come before my love for the other, but I share certain aspects with one, my wife and "primary" more than I do with others.
Does that make sense?