The term that describes the person(s) within a relationship who is not as invested in the relationship as the other member(s). The tertiary will generally expect less in the way of emotional, financial or practical support.
A person will often default into the tertiary role if they cannot offer the other members the time and energy they might be expecting from a partner in a relationship.
Multiple tertiary people will sometimes form a relationship which will be casual by nature. Often the scope of the relationship will be quite short which can be as a result of busy schedules or long distances.
Nathan didn’t feel too bad about heading to the nightclubs even though the people he was in a relationship with were staying home and watching movies. He was the tertiary, after all!