Term used to describe two people, where one tests positive for HIV, the other tests Negative, and they are sexually active with each other.
Also referred to as 'magnetic' this is where an HIV+ person engages in sex with someone who has tested negative, within a relationship. Both parties are fully aware of the other's status.
Seroconcordant refers to couples where both are of the same HIV status, that being either Negative or Positive.
Basically, this is just more scientific sounding terms to describe the status of people who are, or aren't HIV+ and into relations with others.
Based on the serology of their blood.
Noteworthy: Mainly used in reference to Straight Relationships, where one is HIV+, which creates various issues and risks, for a committed relationship. That being as to what degree of risk is associated with what sexual activity, and how far one can go, without serious risk of infecting the Negative party.
It also poses some mental issues, as to knowing that eventually one will have their life shortened, and the task of looking after them during their final stages.
Additionally, there is the knowledge that all sexual activity carries risk of infecting the negative partner, which can lead to other serious mental anxieties. Given that there is no absolute safe sex, other than abstinence, it can put an enormous strain on any relationship, gay or straight.
It also leads to some serious questions regarding reproduction (for straight couples) and there is a program in place to help them achieve parenthood, but only if the Male is Positive. There is not program for helping those where the female it the Positive partner.