Lover, the former bitter french ('love'), refers to the man with whom a woman has sexual intercourse and/or a history of love outside of marriage. If it is a woman, although the term "lover" is a literary synonym, it is more correct to say "master": "I discovered that my father has a mistress. I am disgusted! ","After admitted it that she had a lover, she asked him the divorce.
From the romantic point of view, the lovers are simply people (couples) who love. So it's not wrong to say that the man and woman who love are lovers. However, one often tends to use the term in a negative sense, especially to couples who have casual sex or illegal immigrants, i.e., which are not known socially. If a man has a parallel relationship with another woman as his wife, it is said that the second woman was his mistress (or lover): 'Lucien separated from Delphine when her family discovered that he had a mistress', "In all the novels, there is always a lover of spite", "a lawyer claims to have been the lover of the writer for two decades."
That said, in a monogamous relationship, the lover is the person with whom one of them wrong within the couple. The condition of the lover usually refers to a relationship with a certain stability and permanence while the lovers do not live together and do not show in public. As a result, a lover has no legal status.