Symbiote
From The Stargate Omnipedia
The larval form of beings which often pose as gods.
Utterly helpless, a symbiote requires either a minute electrical current in a liquid habitat, or a living host to survive.
See also: Prim'tah, Goa'uld, Tok'ra
KEY EPISODES
Children of the Gods - Teal'c displays his symbiote to his teammates to show them what it is.
The Enemy Within - Teal'c confirms he is still carrying his symbiote after Major Kawalsky is possessed by one.
The First Ones - SG-1 visits the original homeworld of the Goa'uld, where a primitive symbiote leaps from a lake and nearly reaches Daniel.
Last Stand - Daniel is surprised to see the System Lords partake in a "feast" at their summit, consuming live symbiotes of their own kind.