In China, we call a female person having an affair with a guy who’s already in a relationship, ‘小三’ xiǎo sān， or ‘二奶’ èr nǎi.
However, there are some differences between these two words, and I thought it is worth sharing it with you… don’t ask why lol!
小三 xiǎo sān：
Generated from the word ‘第三者‘ dì sān zhě, which translate it literally, ‘the third person’. It is a neutral word used to describe the third person who entered into and getting involved in other people’s relationship.
On the other hand, 小三 is much more of an insulting word. It describes the woman as to be disgraceful and morally wrong.
二奶 èr nǎi：
二奶 is a term used to describe a woman who is having an affair with a rich and married guy, purely (well， in most cases) because of money. 二奶 can also refer to the second wife of a man, as back in the days Chinese men are allowed to have more than one wife.
We also have a term for someone who is not in a relationship and not having an affair, but because of their good look and single status, they kept attracting unwanted attention from male who are already in a relationship. This term is:
单身公害 dān shēn gōng hài
单身 means single， 公害 public threat
Translate it literally,
a single person who had become a public threat.