Paralithium was a substance which was used as fuel for some ion-based propulsion systems, and which could be converted into trilithium.

In 2373, the USS Voyager detected four vessels in Akritirian space which showed paralithium plasma emissions. Three of them had not produced trilithium, but a cargo vessel which was using this fuel, was later determined to have used their paralithium fuel to create a trilithium explosive, and supplied it to the terrorist organization known as "Open Sky." (VOY: "The Chute")

The authors of the Star Trek Encyclopedia (4th ed., vol. 2, p. 123) speculate, upon the extreme lengths Tolian Soran went to get trilithium in Star Trek Generations, that paralithium technology was unknown in the Alpha Quadrant. However, it was later revealed that paralithium cells were used in Nausicaan technology as early as the mid-23rd century.
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