I have fallen out of favor with using the term secondary, though when I used it I used it to mean, my other relationships (all of their own strengths and flavors and commitments) who were not my primary. I defined primary as the person I make the rules with, the person who is more of a partnership than someone I am dating. I have taken to calling him my primary partner and other people my partners.
I find this unproblematic because it describes the situation. Being poly doesn’t mean all relationships have to have the same parts, “primary” to me means the solid one. The rock. The support. The one I grow with.
The others, they are gifts. Lovers, playmates, friends-I-can-kiss, but they are not the same.
I refuse to say that they will not evolve into primaries, and maybe eventually I will need to find a new word. But I don’t mind acknowledging that different relationships have different power to me.
I also don’t mind that some people would prefer different words or all out free-love style, that’s just not what I happen to have.