unobtainium
A fictional material or component which, if it existed, would enable one to easily solve a hard problem.
countable, uncountable
In order to illustrate some of the features of data processing under ideal conditions, a set of simulated data was generated and is presented in the last four columns of Table II. The hypothetical material is called Unobtainium.
An especially rare material or component that is outrageously expensive or otherwise almost impossible to get hold of.
countable, uncountable
The vacuum furnace has been greatly improved and today we are melting titanium, zirconium, high alloy steels and other materials which are remarkably free of impurities and possess improved properties. These high performance metals are used in turbojet engines, missiles and atomic reactors. […] We call it "unobtainium."
A high technology component that is extremely expensive.
countable, uncountable
Where it comes from
From unobtain(able) + -ium (suffix typical of names of atomic elements), modelled after uranium and titanium. Compare also the origin of dysprosium.
Adapted from the Wiktionary entry unobtainium (CC BY-SA 4.0).