A rare metal alloy, no longer produced by the 23rd century, that was used in the hulls of the first NX-class starships and powers an ancient time portal.
Horonium is a material from the early days of Starfleet. It was used in building the NX-class ships of the 2150s and was later phased out, so that very little of it survives.
It figures in the Strange New Worlds episode Those Old Scientists. A time portal sends Ensign Brad Boimler, and later Beckett Mariner, from the 2380s to Pike's Enterprise, and the device proves to run on horonium, of which there is not enough left to operate it again. An Orion crew who also claim the portal complicate matters.
The solution is found aboard the Enterprise itself: a piece of the NX-01 containing horonium was built into the ship as a tribute, and it provides just enough to send the visitors home.