Unobtainium (or unobtanium) is a term used in fiction, engineering, and common situations for a material ideal for a particular application but impractically difficult or impossible to obtain. Unobtainium originally referred to materials that do not exist at all, but can also be used to describe real materials that are unavailable due to extreme rarity or cost. Less commonly, it can mean a device with desirable engineering properties for an application that are exceedingly difficult or impossible to achieve.
The properties of any particular example of unobtainium depend on the intended use. For example, a pulley made of unobtainium might be massless and frictionless. But for a nuclear rocket, unobtainium might have the needed qualities of lightness, strength at high temperatures, and resistance to radiation damage; a combination of all three qualities is impossible with today's materials. The concept of unobtainium is often applied hand-wavingly, flippantly, or humorously.
The word unobtainium derives humorously from unobtainable, with -ium, a suffix for chemical element names. It predates the similar-sounding systematic element names, such as ununennium, unbinilium, unbiunium, and unbiquadium. An alternative spelling, unobtanium, is sometimes used, by analogy to the names of real elements like titanium and uranium.