As a manager who does a lot of interviews and has run teams with a lot of different working dynamics, culture fit is something I used to describe a collection of social skills and dynamics at play in my team, with the company as a whole, and with our clients. There are a large number of ways people can interact well, and there are a number of specific things an employee might do to make these dynamics strained. If, in the course of an interview, I can identify a large number of these, then I have effectively identified someone who will take up a large portion of my time in coaching & mentoring to smooth out those habits, or who will take up my time in dealing with inter-office conflict. If that's the case, I will turn that person down based on a culture fit and choose another candidate that can present both the technical skills required as well as the soft-skills that allow us all to work more effectively.
This is my interpretation of those terms and how I use it personally. My basis for behavioral traits is based on personal experience as well in working at a large number of companies in various roles, industries, and positions. I don't believe that it is at all discrimination because I'm using my best judgement for who can work most effectively with my team. That's just smart business.