The dybbuk, from Yiddish and Hebrew roots meaning "attachment" or "cleaving", is a concept from Jewish folk mysticism describing the soul of a dead person, one that cannot find rest because of sins committed in life or an unresolved obligation, that possesses a living person's body and speaks through it, usually against its host's will.
The idea developed within Ashkenazi Jewish communities of Eastern Europe from roughly the sixteenth century onward, drawing on earlier Kabbalistic concepts of the soul's fate after death but formalising into a distinct possession narrative complete with its own diagnostic signs, principally a change in the possessed person's voice and the dybbuk speaking through them in the first person, and its own remedy, an exorcism ritual performed by a qualified rabbi, typically involving prayer, the sounding of a shofar and formal command that the spirit depart, sometimes through the smallest toe. Unlike many possession traditions elsewhere, the dybbuk narrative usually gives the possessing soul a specific, nameable grievance or unfinished business, so the exorcism functions partly as a negotiation, establishing why the soul is trapped before commanding it to leave, rather than a purely adversarial expulsion.