The word tauba is derived from ta'b means to come back or return. Thus, tauba basically means return (from sin). The adjective nusuh means sincere, is the companion of the noun tauba. God demands of the believers a "sincere return" (taubatan nusuhan), and He in turn will make them enter paradise (66:8) Once Mu'adh bin Jabal said, "O'Prophet! What is sincere repentance (tauba nusuha)?" The Prophet said, "A man be contrite for the fault that befell him and apologizes to God, then as milk returns not to the udder, he shall not return to it" (Tabari, 1:38). God Himself is described as the accepter of tauba (9:104, 40:3, 42:25). Tauba or repentance is a contrition or regret and self-mortification with the intention of obtaining God's pardon. Repentance can, however, only be accepted as long as one remains a believer: "Those who disbelieve after their believing then increase in disbelief, their repentance (taubatuhum) shall not be accepted and these are they who go astray" (3:90). But the term tauba may denote not just human "return" from sin, but also God's "return" from wrath. This is the case in 4:92, in which a Muslim guilty of unintentional murder is demanded to fulfill some duties, including the payment of blood money, which are imposed on him in order to gain God's tauba.
Another key term is tawwab, which like tauba has a two-fold function. On the one hand, it describes humans who repent repeatedly (2:222), but in most cases it stands for God, Who is willing to accept a human being's repentance. In the verses applying this epithet to God, His merciful response is promised to the Prophet himself (110:3) as well as to Muslims who have acted unjustly towards other Muslims, mainly through slander and spying (24:11, 49:12), or disobedience to the Prophet on legal matters (4:64), or stayed behind the fighting ranks (9:118). Another form connected with repentance is ta'ibun, which designates persons who repent (9:112). This verse provides a list of basic characteristic of the ideal Muslim and the fact that repentance is included in the list means that a believer must always be on guard with respect to his or her unblemished virtues. In many other passages the idea of repentance is conveyed by the verb taba with its various tenses, where again a two-fold function is discernible. On the one hand, taba (with ila) denotes returning from sin to God and, on the other (with ala), it signifies God's returning from wrath to forgiveness. When denoting human repentance, taba is not necessary confined to believers and may also allude to unbelievers acting against the Muslims. In their case, returning to God means simply embracing Islam (19:60, 25:70-1).
The fact that repentance may mean embracing Islam comes out most clearly in the fact that those who have followed the Prophet are called "those who have returned (man taba) (11:112)." The angels beseech God to pardon those who have returned (taba) and followed His way and to save them from the punishment of hell (40:7). Repentance is also offered to the hypocrites, in which case it means restoring their faith to its proper sincerity, otherwise they, too, are condemned to hell (4:145-6, 9:74). The same fate awaits apostates if they do not repent (3:86-9). When referring to the believers, the verb taba means mainly desisting from all kinds of sins against other believers, such as slander (24:4-5, 66:3-4) or finding fault with each other (49:11) or accepting usury (2:278-9). The repentance of the believers is also accepted in cases of unintentional crimes (4:17, 6:54, 16:119).
The believers repentance is considered a constant state of self-trial and improvement, therefore, the need to repent is relevant at all stages of life. For example, one is requested to "return" to God when one is forty years old, i.e. has reached the peak of one's abilities (46:15). In the same vein, God addressed all believers, saying "return (tubu) to God all of you. O believers, so that you may be successful" (24:31). As noted above, the verb taba (with ala) also signifies God's returning from wrath to forgiveness (3:128, 33:24), and His mercy is reserved mainly for believers. For this reason taba may occur in contradistinction to the punishment awaiting the hypocrites and the unbelievers (33:73). It should be observed that there is a mutual dependence between God's mercy as conveyed by the verb taba, and the believer's repentance, which is conveyed by the same verb, which comes out explicitly in 5:39: "Whoever returns (taba) after his iniquity and reforms, then surely God will return to him (yatubu alayhi)." God's mercy is sometimes the first cause that generates repentance, as appears to be the case in 4:26-7: "God desires to explain to you, and to guide you into the ways of those before you, and to return unto you (wa-yatuba alaykum).
The concept of tauba comes out in further passages employing roots synonymous to t-w-b, such as n-w-b, which always occurs in the fourth form (anaba) and denotes return (from sins to God). It is usually employed to describe one's desisting from idolatry and returning to God, so that anaba actually means embracing Islam. Besides, it is stated that God guides towards Himself those who return (anba) (13:27), which means that return to God is the result of God's willing. The same idea recurs in 42:13, which states: "God chooses for Himself whom He pleases, and guides him who returns (yunibu) towards Himself." The root a-w-b, which also means return, features in the sense of repentance in the form awwabin (17:25). The verb raja'a (return) may also occur in the sense of repentance (7:174). Closely associated with the idea of repentance is the idea of desisting from sin, as conveyed by the verb intaha (with an) (4:171).
The Prophet also said, "There is nothing that God loves more than a youth who repents," and "He who repents of sin is even as one who has no sin." The Koran says that God loves penitents (yuhibbu