Yurium was a chemical compound with the chemical formula YU2. This compound was capable of storing large amounts of energy and was usually used as an explosive. A radioisotope byproduct, Jakmanite 632, was created artificially from the decay of high-energy yurium. (TNG: "Hollow Pursuits")
During his scheme to commandeer the USS Discovery using temporal loops, Harry Mudd destroyed the ship in one of the loops using an anicium-yurium reaction. (DIS: "Magic to Make the Sanest Man Go Mad")
In the Soong-type androids, a yurium membrane was used for the construction of the housing for the emergency manual control. (TNG: "The Measure Of A Man")
Bela Tyken used anicium and yurium from his cargo to create a massive explosion to free his ship, which was trapped in a rift in space. (TNG: "Night Terrors")
In 2365, a planetary scan by the USS Enterprise-D detected 2% of yurium in the atmospheric composition of Theta VIII. (TNG: "The Royale")
The suffix -ium is used in chemistry to identify a metallic element. In the real world, Y is the elemental symbol for yttrium and U is the elemental symbol for uranium.