In polyamory circles, people refer to "primary", "secondary", and even sometimes "tertiary" partners. Usually, a 'primary' is the person you've been dating the longest, though it's not uncommon for people to refer to having two primaries (usually someone who lives with both of them), and sometimes people who have recently broken up with a primary and are only pursuing more casual relationships with existing or new secondaries refer to to "being one's own primary".
So I thought it might be interesting to post my own take on the distictions - I am sure anyone who reads this will know that this tiny blog does not begin to speak for the poly community.
I think two people become primaries when they begin thinking about living their lives together (you know, the part of the relationship where you both know that if one of you proposes the other will say 'yes', but nobody's gotten down on one knee yet), which, according to hollywood and soap operas, more or less correlates with the 'moving in together' sort of stage.
I think that even monogamous people do this - they start out as secondaries, and then become primaries later on down the road. Just as you go from 'dating' to 'exclusive', a further step is taken where you become what I would call 'primaries'.
This meshes well with the way that poly people often seem to use "primary" and "secondary" - one's primary partner/s are those one lives with, has a family with, (possibly) raises children with, etc, whilst ones secondary/s tend to be in the "dating" spectrum somewhere.
I know there are many poly people who refer to having, say, a primary of 8 months and a secondary of 2 months; in my mind, depending on the seriousness of both relationships, they'd both fit more to what I feel a "secondary" might be to me.
I also feel like I should mention that I object to the implicit hierachy present in the terms "primary" and "secondary". Lots of people seem to use "SO" and "OSO" (significant other and other significant other), which I think is quite a bit nicer. SO and OSO seem to roughly correspond to the terms 'primary' and 'secondary', but in my opinion seem to be a little bit, well, fairer!