A soul bond is a mystical event rarely experienced on planet Hogwarts (Gamma Germanicus VII), where two beings who are uniquely destined for and compatible with each other form a permanent bond, allowing each component entity of the bond to share the other's memories and abilities.
One such incident occurred on Federation Stardate 50893 (native dating system: April 24th, 1992), when Harold James "Harry" Potter and Hermione Granger, having traveled back in time to correct a temporal incident regarding their relationship, formed a soul bond in the lobby of Gringotts Wizarding Bank.
Due to the unique circumstances behind the bond's formation, the Harry Potter and Hermione Granger of that time period (at the time in their second year of education at Hogwarts School of Witchcraft and Wizardry) became soul bound as well. (Harry Potter and the Fountain of Possibilities: "Chapter 10: 'The Truth Comes Out," "Chapter 11: 'A New Destiny'")
Among the benefits afforded to those in a soul bond are the immediate emancipation of component entities under the age of 17 years (the age of majority among Hogwarts' magical population) and being noted as legally joined in marriage within the magical community. (Harry Potter and the Great Ascension: "Prologue: Year Two [The End]")