Teshuvah תשובה
Return — undoing what was done and changing oneself, possible at any moment.
Teshuvah literally means «return», not «repentance»: not regret for the past but a return to the point where a person was still himself. The sages say teshuvah was created before the world — a world in which a mistake could not be undone could not exist.
The Rambam sets out the sequence: stop the act, regret it, confess aloud, resolve not to repeat. The real test, he says, is the same situation and a different choice. Offences against another person, however, are not atoned by Yom Kippur until forgiveness is asked directly.
The Talmud preserves both sides: «where the returning stand, the wholly righteous cannot stand» — and the warning that whoever says «I will sin and repent» is denied the chance. The ten days between Rosh Hashanah and Yom Kippur are given to this.