Repentance is the activity of reviewing one's actions and feeling contrition or regret for past wrongs, which is accompanied by commitment to and actual actions that show and prove a change for the better. In Islam it is often defined as an action, turning away from self-serving activities and turning to God, to walk in his ways. 
In modern times, it is generally seen as involving a commitment to personal change and the resolve to live a more responsible and humane life. In other words, being sorry for one's misdeeds. It can also involve sorrow over a specific sin or series of sins that an individual feels guilt over, or conviction that he or she has committed. The practice of repentance plays an important role in the soteriological doctrines of Judaism, Christianity, and Islam. Analogous practices have been found in other world religions as well. In religious contexts, it often involves an act of confession to God or to a spiritual elder (such as a monk or priest). This confession might include an admission of guilt, a promise or intent not to repeat the offense, an attempt to make restitution for the wrong, or in some way reverse the harmful effects of the wrong where possible.
In Biblical Hebrew, the idea of repentance is represented by two verbs: שוב shuv (to return) and נחם nacham (to feel sorrow). In the New Testament, the word translated as 'repentance' is the Greek word μετάνοια (metanoia), "after/behind one's mind", which is a compound word of the preposition 'meta' (after, with), and the verb 'noeo' (to perceive, to think, the result of perceiving or observing). In this compound word, the preposition combines the two meanings of time and change, which may be denoted by 'after' and 'different'; so that the whole compound means: 'to think differently after'. Metanoia is therefore primarily an after-thought, different from the former thought; a change of mind and change of conduct, "change of mind and heart", or, "change of consciousness".
The doctrine of repentance as taught in the Bible is a call to persons to make a radical turn from one way of life to another. The repentance (metanoia) called for throughout the Bible is a summons to a personal, absolute and ultimate unconditional surrender to God as Sovereign. Though it includes sorrow and regret, it is more than that. It is a call to conversion from self-love, self-trust, and self-assertion to obedient trust and self-commitment to now live for God and his purposes. It is a change of mind that involves a conscious turning away from wrong actions, attitudes and thoughts that conflict with a Godly lifestyle and biblical commands, and an intentional turning toward doing that which the Bible says pleases God. In repenting, one makes a complete change of direction (180° turn) toward God. The words "repent," "repentance," and "repented" are mentioned over 100 times in the Bible.
Repentance typically requires an admission of guilt for committing a wrong or for omitting to do the right thing; a promise or resolve not to repeat the offense; an attempt to make restitution for the wrong, or in some way to reverse the harmful effects of the wrong or the omission where possible.
says, "Thus saith the Lord GOD; Repent, and turn yourselves from your idols; and turn away your faces from all your abominations."
Rabbinic Jewish literature contains extensive discussions on the subject of repentance. Many rabbinic sources state that repentance is of paramount importance to the existence of this world, so that it was one of the seven provisions which God made before the Creation. "The Holy One, blessed be His name, said to Elijah, 'Behold, the precious gift which I have bestowed on my world: though a man sins again and again, but returns in penitence, I will receive him.' " "Great is repentance: it brings healing into the world"; "it reaches to the throne of the Lord"; "it brings redemption"; "it prolongs man's life"; Talmud Yoma 86a). "Repentance and works of charity are man's intercessors before God's throne". Sincere repentance is equivalent to the rebuilding of the Temple, the restoration of the altar, and the offering of all the sacrifices.
Sincere repentance is manifested when the same temptation to sin, under the same conditions, is ever after resolutely resisted. "He that confesses his sin and still clings to it is likened to a man that holds in his hand a defiling object; though he batheth in all the waters of the world he is not cleansed; but the moment he casteth the defiling object from him a single bath will cleanse him, as it is said: 'Whosoever confesses and forsakes them [his sins] shall have mercy' ".
According to Jewish doctrine, repentance is the prerequisite of atonement. Yom Kippur, the day of atonement, derives its significance only from the fact that it is the culmination of the ten penitential days with which the Jewish religious year begins; and therefore it is of no avail without repentance Though man ought to be penitent every day, the first ten days of every year are the acceptable time announced by the prophet Isaiah: "Seek the Lord while he may be found, call upon him while he is near".
Repentance and the Day of Atonement only absolve one from sins committed against God; from sins against another person they absolve only when restitution has been made and the pardon of the offended party has been obtained.
No one need despair on account of his or her sins, for every penitent sinner is graciously received by God. Jewish doctrine holds that it is never too late, even on the day of death, to return to God with sincere repentance for "as the sea is always open for every one who wishes to cleanse himself, so are the gates of repentance always open to the sinner". Jewish doctrine states that the hand of God is continually stretched out to receive a sinner. One view in the Talmud holds that a repentant sinner attains a more exalted spiritual eminence than one who has never sinned. It is a sin to taunt a repentant sinner by recalling their former sinful ways.
Repentance occupies a prominent position in all the ethical writings of the Middle Ages. Bahya ibn Paquda devotes a special section to it in his 'Hovot ha-Levavot", "Gate of Repentance." Maimonides devotes the last section of "Sefer ha-Madda'" in his Mishneh Torah to the subject. One of the most significant medieval works on Repentance is "Shaarei Teshuva," the "Gates of Repentance" by Rabbeinu Yona of Gerona.
In the Hebrew Bible, repentance generally leads to salvation. In some cases, individuals or nations repent of their sins and are spared God's judgment. Sometimes the punishment avoided is destruction in this life, and sometimes it is damnation. In the Book of Jonah, the prophet initially chose to disobey God's command, and then he repented and became obedient. However, Jonah returned to disobedience when he hoped for the destruction of the city of Nineveh. The Hebrew term teshuvah (lit. "return") is used to refer to "repentance". This implies that transgression and sin are the natural and inevitable consequence of man's straying from God and his laws, and that it is man's destiny and duty to be with God. The Bible states that God's loving-kindness is extended to the returning sinner.
The Torah (five books of Moses) distinguishes between offenses against God and offenses against man. In the first case, the manifestation of repentance consists in: (1) Confession of one's sin before God, the essential part being a solemn promise and firm resolve not to commit the same sin again. (2) Making certain prescribed offerings. Offenses against man require, in addition to confession and sacrifice, restitution in full of whatever has been wrongfully obtained or withheld from one's fellow man, with one-fifth of its value added thereto. If the wronged man has died, restitution must be made to his heir; if he has no heir, it must be given to the priest who officiates at the sacrifice made for the remission of the sin.
There are other manifestations of repentance mentioned in the Bible. These include pouring out water, which symbolizes the pouring out of one's heart before God; prayer self-affliction, as fasting; wearing sackcloth; sitting and sleeping on the ground. However, the Prophets disparaged all such outer manifestations of repentance, insisting rather on a complete change of the sinner's mental and spiritual attitude. "Rend your heart, and not your garments, and turn unto the Lord your God: for he is gracious and full of compassion, slow to anger and plenteous in mercy, and repenteth him of the evil". In , the Bible states that repentance brings pardon and forgiveness of sin. Apart from repentance, no other activities, such as sacrifices or religious ceremonies can secure pardon and forgiveness of sin.
In the New Testament, one of many examples of repentance in the New Testament can be found in the parable of the prodigal son found in . Other instances of repentance included water baptism and restitution.
Repentance appears prominently in the Scriptures. See the description of repentance in the Hebrew Bible above for repentance in the Old Testament. In the New Testament, the first command that Jesus gave was to repent, thus repeating the message of John the Baptist. Jesus sent out disciples who "proclaimed that people should repent". In his Pentecost sermon, Peter the Apostle called on people to repent, an appeal he repeated in his sermon at the Beautiful Gate of the Temple: "Repent therefore, and return again to God, that your sins may be blotted out". Paul the Apostle likewise testified "both to Jews and to Greeks of repentance toward God" and said that "The times of ignorance God overlooked, but now he commands all people everywhere to repent".
The Greek word used for repentance in the New Testament is μετάνοια (metanoia), and the Greek verb for "to repent" is μετανοῶ, contracted from μετανο-έω (metano-eo), as in Mark's account of the initial preaching of Jesus: "The time is fulfilled, and the kingdom of God has come near; repent, and trust in the good news."
In English, the prefix meta can indicate "beyond, about", as "meta-economics" or "meta-philosophy" (see meta), inspired by the non-Greek use of the word "metaphysics", which in Greek was just the title of a work of Aristotle, the Metaphysics, so named simply because in the customary ordering of the works of Aristotle it was the book following the Physics; the Greek word thus meant nothing more than "[the book that comes] after [the book entitled] Physics". In Greek, composite words that have μετα- (meta-) as the initial element are most frequently used "of change of place, condition, plan, etc.", as in the English word "metamorphosis". Even in a non-religious context, the Greek word μετάνοια (metanoia), in particular, meant "change of mind or heart, repentance" or, in rhetoric, "afterthought, correction".
The Augsburg Confession (known in Latin as Confessio Augustana) is the primary confession of faith used in the Lutheran Church. It is one of the most important documents of the Protestant Reformation. It divides repentance into two parts:
In the Calvinist tradition within Protestantism, there is a threefold idea involved in true repentance. The Protestant reformer John Calvin wrote that repentance "may be justly defined to be a true conversion of our life to God, proceeding from a serious fear of God, and consisting in the mortification of the flesh and of the old man, and in the vivification of the Spirit.” He further said that "it will be useful to amplify and explain the definition we have given; in which there are three points to be particularly considered".
In the first place, when we call repentance 'a conversion of the life to God', we require a transformation, not only in the external actions, but in the soul itself; which, after having put off the old nature, should produce the fruits of actions corresponding to its renovation....
In the second place, we represented repentance as proceeding from a serious fear of God. For before the mind of a sinner can be inclined to repentance, it must be excited by the knowledge of the Divine judgment.
It remains for us, in the third place, to explain our position, that repentance consists of two parts—the mortification of the flesh and the vivification of the spirit.... Both these branches of repentance effects our participation of Christ. For if we truly partake of his death, our old man is crucified by its power, and the body of sin expires, so that the corruption of our former nature loses all its vigor.... If we are partakers of his resurrection, we are raised by it to a newness of life, which corresponds with the righteousness of God." [Quotes from A Compend of the Institutes of the Christian Religion by John Calvin edited by Hugh T. Kerr, The Westminster Press-Philadelphia 1939.]
The word tawbah (repentance) in Arabic literally means 'to return', and is mentioned in the Quran. In an Islamic context, it refers to the act of leaving what Allah has prohibited and returning to what he has commanded. The act of repentance can redeem the sins and give the opportunity to go to heaven:
Although repentance is considered as one act that can be used for cleansing the sins, the Quran notes that all of the sins are forgiven with or without repentance except the state of Shirk, i.e. the act of worshiping another deity besides Allah. The believing person must repent for shirk sin and seek the forgiveness of Allah. Two verses are giving comments regarding Shirk, An-Nisaa 4:48 and 4:116.
Islam does not accept the concept of original sin; instead, it teaches that a person is born in a state of innocence and pure belief. The person remains in that state of sinlessness until reaching the age of puberty, after which he is accountable for his sins.
The Buddha considered shame over doing wrong (Pali: hiri) and fear of the consequences of wrongdoing (Pali:otappa) as essential safeguards against falling into evil ways and further as extremely useful in the path of purification. Also recommended was the regular practice of self-assessment or wise reflection (Pali: yoniso manasikara) on one's own actions in relation to others and the bigger picture.
For all the evil deeds I have done in the past
Created by my body, mouth, and mind,
From beginningless greed, anger, and delusion,
I now know shame and repent of them all. 
Hoʻoponopono (ho-o-pono-pono) is an ancient Hawaiian practice of reconciliation and forgiveness, combined with (repentance) prayers. Similar forgiveness practices were performed on islands throughout the South Pacific, including Samoa, Tahiti and New Zealand. Traditionally hoʻoponopono is practiced by healing priests or kahuna lapaʻau among family members of a person who is physically ill. Modern versions are performed within the family by a family elder, or by the individual alone.