Teshuvah means return, and it is the Jewish word for repentance, not primarily remorse but a turning back to God and to the way one had left. The prophets supply its foundation: "Return, O Israel, to the LORD your God" (Hosea 14:2); "let the wicked forsake his way and the unrighteous man his thoughts, and let him return to the LORD, and he will have mercy on him" (Isaiah 55:7); and Ezekiel's insistence that God has no pleasure in the death of the wicked but desires that he turn from his way and live (Ezekiel 18:23, 33:11).
Maimonides gave the classical analysis in the Laws of Repentance in the Mishneh Torah: confession in words, abandonment of the act, regret for having done it, and resolve not to return to it. His test for complete repentance is exacting and much quoted, that the person, placed again in the same circumstances with the same opportunity and the same desire, refrains. He also states the rule that governs a great deal of practice: offences against another person are not forgiven by God until the injured party has been appeased, which is why the days before the Day of Atonement are occupied with asking forgiveness of people rather than only of God.
Teshuvah is built into the calendar rather than left to occasion. The month of Elul, the ten days from Rosh Hashanah to Yom Kippur, and the Day of Atonement itself form an annual sequence, and the liturgy of Yom Kippur is organised around confessions recited in the plural, so that the community acknowledges together what individuals may or may not have done.
The tradition holds the door remarkably wide, teshuvah is said to have been created before the world, and to reach where nothing else reaches, while refusing to let it be gamed. One who says "I will sin and then repent" is not given the opportunity to repent, and the same passage states that the Day of Atonement covers offences between a person and God but not those between a person and their fellow until the fellow has been appeased (Mishnah Yoma 8:9). In modern usage baal teshuvah, master of return, names a Jew who has taken on observance, and the return-to-observance movements of the later twentieth century took their name from the term.