A substance formerly used in medical containment fields which is invisible to standard scans and which caused a string of baffling malfunctions aboard the Enterprise-D.
Invidium is the hidden cause of the trouble in the episode Hollow Pursuits. The Enterprise-D is carrying tissue samples from the Mikulaks to treat an epidemic when unrelated systems start to fail: an antigrav unit, a glass that leaks in Ten Forward, a transporter, and finally the engines, which accelerate out of control.
La Forge's team can find no link until Reginald Barclay suggests looking at what the people involved had touched rather than at the machines. The trail leads to a broken sample canister.
Invidium, no longer used by the Federation, changes the molecular structure of materials it touches and is carried on hands and tools. It does not register on sensors. Once identified, it is made inert by flooding the affected injectors with extreme cold.