In Jewish mysticism and folklore, a dybbuk is an evil spirit which enters a person, takes over their personality, precipitates mental illness, speaks through their voice and generally causes trouble. 

Dybbukim first appear in talmudic writings. According to early views they were devils or demons. Later mystics posited that dybbukim could be the spirits of dead sinners, who were so evil in their lifetimes that their souls could find no resting place in the world to come and were therefore compelled to take refuge in the bodies of innocent living victims.

Kabbalistic works, at least from the 16th century onwards, sometimes contain instructions and protocols for the exorcism of dybbuks, ceremonies to drive them out of the bodies they have colonised.

The dybbuk is short for the dybbuk m'ruach ha- ra'ah, meaning "cleaving from the evil spirit." The word dybbuk itself is from the Hebrew "davak," which means to "stick" or "cleave." (Devek means "glue" in modern Hebrew.)

In the Torah, devekut, "cleaving to God," is a central goal of Judaism. Deuteronomy 11:22 commands us to walk in all God's ways and, cleave (davak) to him. Kabbalah and Chasidut understood devekut (meaning something like continuous awareness of the presence of God) to be the main goal of our spiritual lives.

So a dybbuk is an unwanted adhesion to a person's soul.

The dybbuk entered popular culture with Shlomo Anski's 1916 Yiddish play (later a film) of that title.