Adultery (Zinain Arabic, also called philandery or "Fornication") is a form of extramarital sex. To carry on a sexual affair, especially an extramarital affair, with a woman one cannot or does not intend to marry. Used of a man. 
Definition according to Webster dictionary: voluntary sexual intercourse between a married man and someone other than his wife or between a married woman and someone other than her husband. 
In Islamic Law, it is defined as sexual intercourse between a man and a woman who is neither his wife nor his slave. Islamic law considers this prohibition to be for the protection of men and women and for the respect of marriage.
Ibn Hajar al-Haythami (May Allah have mercy on Him) said: “Thus it is also known that there are different categories of adultery. If it is done with a non-mahram woman who has no husband it is a serious matter. It is more serious if it is with a non-mahram woman who has a husband; it is more serious still if it is with a mahram. The adultery of a previously-married person is worse than adultery of a virgin, which is indicated by the fact that the punishment is different. And the adultery of an old man who is of sound mind is worse than the adultery of a young man, and the adultery of a free man or a knowledgeable man is worse than the adultery of a slave or one who is ignorant. Al-Zawaajir ‘An Iqtiraaf Al-Kabaa’ir (2/138)
Allah (Glory be to Him) Said in Qur’an: And do not approach unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way. Qur’an.Surah Isra 17:32. By this ayath we can conclude that it is a major sin in Islam, and an evil way. This ayath shows the gravity of the evil, Allah (Glory be to Him) is commanding the humans not to go near adultery. And those who do not invoke with Allah another deity or kill the soul which Allah has forbidden [to be killed], except by right, and do not commit unlawful sexual intercourse. And whoever should do that will meet a penalty. Quran 25:68 
Tafseer Ibn Kathir’s Tafseer on Qur’an Surah Isra 17:32 is given below
Imam Ahmad (May Allah have mercy Him) recorded from Abu Umamah (May Allah be pleased with Him) saying that a young man came to the Prophet (May Allah honour Him and Grant Him peace)and said, "O Messenger of Allah! Give me permission to commit Zina or adultery (unlawful sex).'' The people surrounded him and rebuked him, saying, "Stop! Stop!''
But the Prophet (May Allah honour Him and Grant Him peace) said, (Come close). The young man came to him, and he said, (Sit down), so he sat down. The Prophet (May Allah honour Him and Grant Him peace) said, (Would you like it (unlawful sex) for your mother)! He said, "No, by Allah, may I be ransomed for you.''
The Prophet (May Allah honour Him and Grant Him peace) said, Neither do the people like it for their mothers. The Prophet said, (Would you like it for your daughter)? He said, "No, by Allah, may I be ransomed for you.''
The Prophet (May Allah honour Him and Grant Him peace) said, Neither do the people like it for their daughters. The Prophet said, (Would you like it for your sister)? He said, "No, by Allah, may I be ransomed for you.''
The Prophet (May Allah honour Him and Grant Him peace) said, Neither do the people like it for their sisters.
The Prophet (May Allah honour Him and Grant Him peace) said, (Would you like it for your paternal aunt)? He said, "No, by Allah, O Allah's Messenger! May I be ransomed for you.'' The Prophet said (May Allah honour Him and Grant Him peace), Neither do the people like it for their paternal aunts.
The Prophet (May Allah honour Him and Grant Him peace) said, Neither do the people like it for their maternal aunts. Then the Prophet put his hand on him and said, O Allah, forgive his sin, purify his heart and guard his chastity. After that the young man never paid attention to anything of that nature.  
Narrated Ikrima from Ibn Abbas(R): Allah's Apostle (May Allah honour Him and Grant Him peace) said, "When a slave (of Allah) commits illegal sexual intercourse, he is not a believer at the time of committing it; and if he steals, he is not a believer at the time of stealing; and if he drinks an alcoholic drink, when he is not a believer at the time of drinking it; and he is not a believer when he commits a murder." 'Ikrima said: I asked Ibn Abbas(R), "How is faith taken away from him?" He said, Like this," by clasping his hands and then separating them, and added, "But if he repents, faith returns to him like this," by clasping his hands again, Sahih Al Bukhari Vol 8:800B
The Prophet (May Allah honour Him and Grant Him peace) said: “If a man commits zina, faith will come out of him until it becomes like a cloud over him, and when he stops, his faith comes back to him.” Narrated by Abu Dawud, 4690; al-Tirmidhi, 2625; classed as saheeh by al-Albaani in Saheeh Abi Dawud. 
Whoever falls into adultery must hasten to repent to Allah (Glory be to Him) and conceal himself with His concealment, and not disclose what he has done, because the Prophet (May Allah honour Him and Grant Him peace) said: “Avoid this filthy thing that Allah (Glory be to Him) has forbidden, and whoever commits any sin of that nature let him conceal himself with the concealment of Allah.” Narrated by Al-Bayhaqi and classed as sahih by Al-Albaani in Al-Silsilah al-Sahihah 663.
The expiation for committing adultery with a married woman or any other is sincere repentance that fulfils all the conditions of repentance. That involves giving up the sin completely, regretting what one has done and resolving never to go back to it.
Narrated from Ibn ‘Abbas(R) that some people from among the mushrikeen who had killed and killed a great deal, and committed zina and done it a great deal came to Muhammad (May Allah honour Him and Grant Him peace) and said: “What you are saying and calling us to is good, if only you could tell us that there is any expiation for what we have done.” Then the words “And those who invoke not any other Ilaah (god) along with Allah, nor kill such person as Allah has forbidden, except for just cause, nor commit illegal sexual intercourse” Qur’an.Surah Furqaan 25:68 and “Say: O ‘Ibaadi (My slaves) who have transgressed against themselves (by committing evil deeds and sins)! Despair not of the Mercy of Allaah” Qur’an.Surah Zumar 39:52 were revealed.Sahih Al Bukhari 4436 and Sahih Al Muslim 174 
There are many ways a Muslim can protect them self from adultery
From amongst the most important of means is remembering Allah. Know that the one who is away from the remembrance of Allah will be weak and is susceptible to the attacks of Satan.
Regular fasting is another way of suppressing the sexual desire, as Allah’s Apostle ﷺprescribed it for the young people who cannot afford marriage.
Another way to keep away from adultery is by reminding yourself of how you would hate and dislike it for your mother, daughter, sister, and aunt; this was how the Prophet ﷺput it to a man who came seeking permission for adultery. Remember that Satan watches and waits for you to be distanced from your Muslim brethren, then he makes his attack. And so from amongst the things that will deter you from adultery, is to always be in the company of the righteous, those people that will remind you of Allah.
Remember that saying “No” to someone who invites you to commit adultery will secure you a place of shelter under the shade of Allah’s throne on the Day when their will be no shelter except His. Sahih al-Bukhari 1423 (Vol. 2:504) Abu Hurayrah (R) reported that Allah’s Messenger ﷺsaid: “Whoever Allah protects from the evil of what is between his jaws (i.e. his tongue) and the evil of what is between his legs (i.e. his private parts), he will enter Jannah ( Paradise). Jami` at-Tirmidhi Vol.4:2409