"second wives" (er laopo), women [who may have family or kids but] who indulge in extramarital affairs with men, married or not.
Then we have "the thirds" (disanzhe) who are casual love affairs only.
The queen of all female roles, however - in direct competition with the faithful "wife" (laopo)- is the "mistress" (qingren).
Technically, only married men can have mistresses; otherwise, if the gentleman is single, we would refer to his female company - however many of them- as simple "girlfriends" (nvpengyou).
The Chinese tradition of maintaining mistresses is based on what good Christians would refer to as adultery - a sin; yet in China it is mere custom - a habit.
Consequently, when Westerners first come to China, they are utterly perplexed by the strict division here between marriage, romance, and sex - for which, in Chinese thinking, of course (at least) three different types of women are required.
It has been observed that many Chinese women opt out of the Chinese tradition of cheating husbands and try to find a foreigner, preferably from a traditional monogamous society like Western Europe. Those "foreigners" (laowai) may also cheat on their spouse, of course, but for individual reasons, not, as is the case in China, as a social prescription or norm.