unobtanium

In fiction, engineering, and thought experiments, unobtainium is any hypothetical, fictional, or impossible material, but it can also mean a tangible but extremely rare, costly, or reasonably unobtainable material. Less commonly, it can refer to a device with desirable engineering properties for an application, but which are exceedingly difficult or impossible to achieve.
The properties of any particular example of unobtainium depend on the intended use, e.g. a pulley made of unobtainium might be massless and frictionless; however, if used in a nuclear rocket, unobtainium might have the needed qualities of lightness, strength at high temperatures, and resistance to radiation damage, having all 3 qualities being nearly impossible for today's materials. The concept of unobtainium is often applied hand-wavingly, flippantly, or humorously.
The word "unobtainium" derives humorously from 'unobtainable' with the suffix -ium, the conventional designation for a chemical element. It pre-dates the similar-sounding IUPAC systematic element names, such as ununennium. An alternative spelling, unobtanium is sometimes used based more closely to the spelling of actual materials such as titanium or uranium.

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