Wiktionary
unobtanium
n. 1 A fictitious substance, which, if it existed, would enable one to solve an intractable problem with ease. 2 An especially rare component that is almost impossible to get hold of. 3 A high tech component that is extremely expensive.
Usage examples of "unobtanium".
All you needed was the right type of unobtanium to hand in order to make it work.
Sort of unobtanium you need in ‘mergency, stuff you can’t find easily.