A rare and highly unstable compound, too volatile to be sent by transporter, that is the only known means of neutralising tricyanate contamination in a water supply.
Hytritium drives the plot of the episode The Most Toys. The water supply of the colony on Beta Agni II has been poisoned with tricyanate, and the Enterprise-D obtains the only available hytritium from the trader Kivas Fajo aboard his ship, the Jovis.
Because the substance cannot safely be beamed, Data ferries it across by shuttlecraft in several loads. On the last trip the shuttle explodes, and the crew, grieving, hurry on to the colony believing him destroyed.
In fact Fajo has kidnapped Data for his private collection. The deception unravels when La Forge and Wesley Crusher examine the records, and when the tricyanate at the colony turns out to have been introduced deliberately, in exactly the amount Fajo's hytritium could cure.