Short answer
Several Arabic words are used. The most common are dhanb (a sin, plural dhunub), ithm (wrongdoing), ma'siyah (disobedience), khati'ah (a wrong or mistake) and sayyi'ah (a bad deed, the opposite of hasanah). Major sins are called kaba'ir and minor sins sagha'ir.
A sin is any act that goes against Allah's command. Hanafi fiqh calls what is forbidden by decisive proof haram and what is forbidden by less decisive proof makruh tahrimi; doing either is sinful, and leaving a fard or wajib act is sinful too.
The four schools agree on this.
Explanation
Dhanb is the general word for a sin, like a tail that follows a person. The believers pray: "Our Lord, indeed we have believed, so forgive us our sins (dhunubana)" (3:16). Ithm is a wrong that holds a person back from good. Allah says: "cooperate in righteousness and piety, but do not cooperate in sin (ithm) and aggression" (5:2). The Prophet ﷺ defined it beautifully: sin is what unsettles your heart and you would hate people to find out about.
Ma'siyah means disobedience, from the verb used in "Adam disobeyed ('asa) his Lord" (20:121). Khati'ah (plural khataya) is a wrong or fault: "Because of their sins (khati'atihim) they were drowned" (71:25). Sayyi'ah is a bad deed, the opposite of hasanah: "good deeds do away with misdeeds (sayyi'at)" (11:114). Other words include jurm (crime), fahishah (indecency) and zulm (wrongdoing, including against oneself).
In fiqh, sins are spoken of by the ruling of the act: haram, makruh tahrimi, or leaving a fard or wajib. In creed, they are divided into major (kaba'ir) and minor (sagha'ir). The opposite words are hasanah (a good deed), birr (righteousness) and ta'ah (obedience).
Repentance is called tawbah, and seeking forgiveness istighfar. Read what are the major sins?, tawbah and the Islamic terms glossary.
The evidence
Quran, Surah Aal-i-Imraan 3:16
ٱلَّذِينَ يَقُولُونَ رَبَّنَآ إِنَّنَآ ءَامَنَّا فَٱغْفِرْ لَنَا ذُنُوبَنَا وَقِنَا عَذَابَ ٱلنَّارِ ﴿١٦﴾
Those who say, "Our Lord, indeed we have believed, so forgive us our sins and protect us from the punishment of the Fire,"
Uses dhunub for sins.
Quran, Surah Al-Maaida 5:2
يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ لَا تُحِلُّوا۟ شَعَٰٓئِرَ ٱللَّهِ وَلَا ٱلشَّهْرَ ٱلْحَرَامَ وَلَا ٱلْهَدْىَ وَلَا ٱلْقَلَٰٓئِدَ وَلَآ ءَآمِّينَ ٱلْبَيْتَ ٱلْحَرَامَ يَبْتَغُونَ فَضْلًۭا مِّن رَّبِّهِمْ وَرِضْوَٰنًۭا ۚ وَإِذَا حَلَلْتُمْ فَٱصْطَادُوا۟ ۚ وَلَا يَجْرِمَنَّكُمْ شَنَـَٔانُ قَوْمٍ أَن صَدُّوكُمْ عَنِ ٱلْمَسْجِدِ ٱلْحَرَامِ أَن تَعْتَدُوا۟ ۘ وَتَعَاوَنُوا۟ عَلَى ٱلْبِرِّ وَٱلتَّقْوَىٰ ۖ وَلَا تَعَاوَنُوا۟ عَلَى ٱلْإِثْمِ وَٱلْعُدْوَٰنِ ۚ وَٱتَّقُوا۟ ٱللَّهَ ۖ إِنَّ ٱللَّهَ شَدِيدُ ٱلْعِقَابِ ﴿٢﴾
O you who have believed, do not violate the rites of Allah or [the sanctity of] the sacred month or [neglect the marking of] the sacrificial animals and garlanding [them] or [violate the safety of] those coming to the Sacred House seeking bounty from their Lord and [His] approval. But when you come out of ihram, then [you may] hunt. And do not let the hatred of a people for having obstructed you from al-Masjid al-Haram lead you to transgress. And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty.
Uses ithm for sin.
Sahih Muslim 2553
Sahih... وَالإِثْمُ مَا حَاكَ فِي صَدْرِكَ وَكَرِهْتَ أَنْ يَطَّلِعَ عَلَيْهِ النَّاسُ ...
"Sin is what unsettles your heart and you would hate people to find out about."
The Prophet's definition of ithm.
Where the ruling is recorded
- Mufradat Alfaz al-Quran (al-Raghib al-Isfahani), Entries for dhanb, ithm and khati'ah. Explains the meanings of the Quran's words for sin.
- Radd al-Muhtar (Ibn Abidin), Book of prayer, on what is disliked in prayer. Explains makruh tahrimi as close to haram and sinful to do.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.