A person (traditionally female, but in today's equal opportunity world; could be male) who maintains a sexual relationship with a person that has a formal or socially recognized sexual partner.
To qualify as a sidepiece, there must be awareness of the other person's primary relationship. The side relationship should also be a secret, at least from the main partner.
The side relationship is often understood to just be about sex. It could be transactional, or purely for the physical pleasure.
Society's default view of a side relationship is a negative one of both parties. I think that is stupid because the world's insistence on sexual monogamy is outdated and thus misguided. All the biological and cultural reasons for monogamy can be reduced to fear of unplanned pregnancies and disease. Since technology has provided solutions to both issues, the necessity for monogamy is gone and people should be allowed to have sex for pleasure. The same way that people are allowed to eat in restaurants or at other people's homes even if their spouse is a chef.
Side Quest 2024.