The internal abdominal pocket of a joined Trill in which the symbiont lives, linked to the host's nervous system so closely that removing the symbiont after joining is normally fatal to the host.
Trill hosts carry their symbionts inside the body, in a pouch-like cavity in the abdomen. Once a symbiont has been placed there its nervous system knits together with the host's, and within a few days the two are biologically dependent on each other. Isoboramine, a neurotransmitter, mediates the link.
Deep Space Nine returns to this anatomy several times. In Invasive Procedures the unjoined Trill Verad forces Doctor Bashir to cut the Dax symbiont out of Jadzia and implant it in himself, leaving her dying until it is restored. In Equilibrium a fall in Jadzia's isoboramine level threatens the joining, and in Tears of the Prophets and Image in the Sand the symbiont is removed from the mortally wounded Jadzia and later placed in Ezri Tigan.
The Next Generation episode The Host showed an earlier version of the idea, with the symbiont Odan carried temporarily by Commander Riker.