A Dybbuk is a fan-used term derived from a reference by Dimo when describing the situation with Agatha. It is called an by Gil. In myth the dybbuk is a disembodied entity that attempts to transfer its personality,usually forcibly, into another's body without removing the resident mentality and psyche. It will hijack the hosts body at will if not suppressed by some means.
Dimo mentions this particular phenomenon, that The Other is , while meeting with the Jägergenerals. The term dybbuk is from Judaism; it is a partial spirit or personality imposed on a victim for some reason, usually a poorly kept ward on the outer doors of one's domicile, thus allowing the dybbuk access to one's home and one's person.
In the Girl Genius Universe this phenomenon is physically real, and is accomplished by using highly developed machinery for transferring a consciousness from one body to another. We do not know if this machine is the accomplishment of Lucrezia Mongfish alone, or whether The Other inspired her. It could be either or both.
This creates a pocket in the victim's mind wherein the dybbuk resides and slides out to take over the victim's body whenever it is strongly motivated by some external stimulus--temptation usually. This applies to the entities seen in the comics they are copies of the template personalities carrying traits, memories, and skills, that come to prominence whenever the opportunity presents itself. This is usually from the one case we have seen during the initial overlay, but the host personality can force itself to the surface and the dybbuk can be suppressed with the right devices. The subject will beof the dybbuk's whenever it is in control.
There are three known dybbuks so far in this story. One is Lucrezia-in-Agatha. The other is Lucrezia-in-Zola. The third is Klaus-in-Gil. Once a dybbuk has infected a character, any actions that character takes while the "parasite" is in control are treated as though that victim-parasite combination is a separate character, which it is for all practical intents and purposes. While "Lunevka" does not infect an existing character it was created by a similar process and is recorded here as one based on this.
Agatha is able to use Heterodyne signals, which members of her family have used to improve their concentration for years, to effectively suppress her dybbuk, and later built herself an army of Dingbots to hum one for her all the time. Later, she had her distress beacon also emit a heterodyne tune. Later, Lucrezia-In-Agatha acquired Agatha's Locket, not knowing Barry had originally made it to suppress Agatha's spark when she was growing up, it instead suppressed Lucrezia-In-Agatha when she put it on.