A round object some species have surgically inserted into their abdomens that creates a personal life link to someone they care for. When the disc no longer functions it means that the Life Force of the other person has terminated.
(Definition from Scifi.com's FarScape site) Surgical implant that creates a permanent link between the lifeforces of two entities. Each entity receives one of a pair of bonded Life Discs; so long as both entities remain alive, both Life Discs will remain active. If one of the bonded entities dies, the Life Disc possessed by the other entity will go dark, signifying the other's passing.
Farscape DVD 2.4 Definition: A surgical implant that creates a permanent lifelink between two entities.