Zina denotes copulation with other than one’s legal spouse (permanent or temporary), which the Quran labels as a grave sin, about which Allah has said: “Do not approach fornication. It is indeed obscene and an evil way.”,
If this act is committed by a married man or woman, it will be called ‘zina muhsanah’.
For information on the relevant rulings and negative outcomes of this ugly act, please follow these links:
What is Zina?, 8243 (site: 8288).
Executing the Hadd and repentance in adultery, 7159 (site: 7508).
Adultery being the trampled right of Allah or people, 9716 (user).
 Israa’:32 “وَ لا تَقْرَبُوا الزِّنى إِنَّهُ كانَ فاحِشَةً وَ ساءَ سَبِيلًا”. Regarding the impermissibility of adultery, see: Furqan:68 and 69; A’raaf:33; An’aam:151; Kulayni, Yaqub, translated by Mustafawi, Sayyid Jawad, Usul Kāfī, vol. 3, p. 391, Wafaa’ Press, 2004; Najafi, Muhammad Hasan, Hurr Amili, Wasa’il al-Shia, vol. 28, book of hudud; Jawaahir al-Kalaam, vol. 41, pp. 258 and 260, Daar Ihyaa’ al-Turaath al-Arabi, Lebanon, 1981.
 Adopted from Question 6787 (user).
 Adopted from Question 7159 (site: 7508). Of course, if a man commits this act, he will be referred to as muhsan, and the adultery of a man and woman in general is referred to as zina muhsanah.