Ibbur (Hebrew: עיבור, "pregnancy" or "impregnation" or "incubation"), is one of the transmigration forms of the soul and has similarities with Gilgul neshamot.
Ibbur is always good or positive, while dybbuk (Yiddish: דיבוק), is negative.
It happens when a righteous soul decides to occupy a living person's body for a time, and joins, or spiritually "impregnates" the existing soul.
The reason for Ibbur is always benevolent—the departed soul wishes to complete an important task, to fulfil a promise, or to perform a mitzvah (a religious duty) that can only be accomplished in the flesh.
In Lurianic Kabbalah, ibbur occurs when an incomplete soul which cannot achieve tikun is completed by the addition of the soul of a tzadik,[1] or spiritual master.