There's a lot of input here by people who seem to find the word "secondary" to be negative.
Perhaps those people should really evaluate themselves and their relationships before attempting to put a term to it.
I have, willingly, been a secondary. We used that term among ourselves. I don't think it was a negative thing at all. That is, of course, because I WANTED to be the non-primary partner in that relationship. "Below priority of the primary" was what my role in that relationship was, and it's the role I wanted at the time.
There's the inverse of this discussion. If the term "secondary" implies "less affection" or "lacking" then perhaps the people who want equality should stop venerating (and of course, HAVING) primaries. You can't have it both ways. Either you elevate one (your primary) or you treat them as equals, in which case nobody is primary or secondary.
I think secondary is a good term. It can be a great place to be as well. The term is descriptive of the situation. If the term applies, and causes discomfort, people shouldn't change the term. They should strike at the root and fix the discomfort.
Connection is the path to passion. Passion is not the path to connection.