Involves an exceedingly rare, valuable or inexplicable substance (or possibly device) that often has so many (often unpredictable) uses that everyone wants it, and it's (of course) important for the plot.
Alternate names: Unobtainium, Handwavium, Eludium, Wishalloy
The first video game about Phlebotinum was released on August 31, 1995.
Real world examples would be water and petroleum (oil) if there ever was scarcity for them (current future predictions indicate there will be in a few hundred years). As these are used in pretty much everything there is (and if there's something that doesn't contain either, you can bet these were used in the production of whatever that is somewhere along the line).