In
polyamory this
term is used to
refer to a
romantic and/or
sexual partner with whom one has a particularly
strong bond. It contrasts with
secondary partner.
Many polyfolk are uncomfortable with the idea or primary and secondary partners because they don't like the hierarchy it imposes, and because they feel that a secondary is like a second class citizen.
Others are fond of the term because they find the hierarchy matches their actual feelings and/or behavior and communicates clearly that the secondary is in fact not as important as the primary.
I've tried poly relationships with and without an explicit hierarchy and I can't say that either of them works particularly better for me, so I just use whatever terms provide the best map of my current situation.