المعنى الاصطلاحي

Male and female friends, who secretly have sexual relations with each other.

الشرح المختصر

"Akhdān" (secret lovers/paramours) is one of the invalid types of marriages that Islam has eliminated, which is called "nikah al-khidn": (to fornicate with a girlfriend/boyfriend in secret). There were people from among the Arabs who used to consider the public commission of fornication as unlawful while viewing what was secret thereof as lawful. They did not ban the act of taking a secret lover, whether it was done by a man or a woman. Islam came to demolish this type of illegal relationship. "Khidn" means a friend, regardless of whether it refers to someone whom one meets directly, over the phone, via internet chat, or by any other means of communication used unlawfully by men and women.

التعريف اللغوي المختصر

"Akhdān" (sing. khidn): a secret friend, a friend that one confides and speaks with, a friend in general. It is mostly used for someone who befriends another on account of sexual desire. The origin of "mukhādanah" is companionship. Other meanings: lover, mate.