A woman (single or married) who successfully seduces a married man and has a romantic relationship with him, yet she does not nessessarily need to be having sex with him. The key to making her a Swiss Mistress is that she is not looking for any long term committment and enjoys being the Mistress. The term came from business class men in Zürich, Switzerland who would often be seduced by women in their business relations and often go to them as a source of lust and sexual release.
A female co-worker flirts with a man at work until he flirts back to the point that they make-out in his car after work. Since she does not want to destroy his marriage and values their friendship, she continues to offer loving confort when he needs it. Sometimes sex. She would be his Swiss Mistress.