Unobtainium

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-waving, 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.
Engineering origin.
Since the late 1950s, aerospace engineers have used the term "unobtainium" when referring to unusual or costly materials, or when theoretically considering a material perfect for their needs in all respects, except that it does not exist. By the 1990s, the term was in wide use, even in formal engineering papers such as "Towards unobtainium [new composite materials for space applications]."
The word "unobtainium" may well have been coined in the aerospace industry to refer to materials capable of withstanding the extreme temperatures expected in re-entry. Aerospace engineers are frequently tempted to design aircraft which require parts with strength or resilience beyond that of currently available materials.
Later, 'unobtainium' became an engineering term for practical materials that really exist, but are difficult to get. For example, during the development of the SR-71 Blackbird spy plane, Lockheed engineers at the "Skunk Works" under Clarence "Kelly" Johnson used 'unobtainium' as a dysphemism for titanium. Titanium allowed a higher strength-to-weight ratio at the high temperatures the Blackbird would reach, but its availability was restricted because the Soviet Union controlled its supply.
Contemporary popularization.
By 2010, the term had diffused beyond engineering, and now can appear in the headlines of mainstream newspapers, especially to describe the commercially useful rare earth elements (particularly terbium, erbium, dysprosium, yttrium, and neodymium). These are essential to the performance of consumer electronics and green technology, but the projected demand for them so outstrips their current supply that they are called "unobtainiums" within the ore industry and by commentators on the US Congressional hearings into the supply security of rare-earths.
'Unobtainium' has come to be used as a synonym for "unobtainable" among people who are neither science fiction fans nor engineers to denote an object that actually exists, but which is very hard to obtain either because of high price (sometimes referred to as "unaffordium") or limited availability. It usually refers to a very high-end and desirable product. Examples include rear cassettes in the mountain biking community, parts that are no longer available for old-car enthusiasts, parts for reel-to-reel audio-tape recorders, or rare vacuum tubes such as the 1L6 or WD-11 that can now cost more than the equipment in which they were fitted.
There have been repeated attempts to attribute the name to a real material. The space elevator research has long used "unobtainium" to describe a material with the necessary characteristics, but carbon nanotubes might make this a real material. The eyewear and fashion wear company Oakley, Inc. also frequently denotes the material used for many of their eyeglass nosepieces and earpieces, which has the unusual property of increasing tackiness and thus grip when wet, as unobtanium.
Science fiction.
"Unobtainium" can refer to any substance that is needed to build some device critical to the plot of a science fiction story but which does not exist in the universe as we know it. For example, a hull material that gets stronger by absorbing and converting heat and pressure into energy in the film "The Core" (2003) was nicknamed "unobtainium". The same concept under different names can be seen in the anti-gravity material cavorite from H. G. Wells' 1901 novel "The First Men in the Moon", as well as the super-strong material scrith from Larry Niven's novel "Ringworld", which requires a tensile strength (i.e., chemical bonds) on the order of the force binding an atomic nucleus. In Frank Herbert's Dune universe, melange (also known as "spice") is a drug required for interstellar travel and is by far the most valuable commodity in the known universe.
The term was also used in James Cameron's 2009 movie "Avatar", as a substance that was named "unobtanium". In the film, it was mined on the fictional moon Pandora and was a room-temperature superconductor; its engineering value allowed for viable interstellar space travel.
Similar terms.
The term handwavium (suggesting handwaving) is another term for this hypothetical material, as are buzzwordium, impossibrium, raritanium, and hardtofindium.
The related term phlebotinum (a derisive term, drawn from "phlegm") is used to describe a technological plot device created by a science fiction writer to "magically" advance the plot to a desired point.
The term eludium (also spelled with variants such as illudium) has been used to describe a material which has "eluded" attempts to develop it. This was mentioned in several Looney Tunes cartoons, where Marvin the Martian tried (unsuccessfully) to use his "Eludium Q-36 Explosive Space Modulator" to blow up the Earth.
Another largely synonymous term is wishalloy, although the sense is often subtly different in that a wishalloy usually does not exist at all, whereas unobtainium may merely be unavailable.
A similar conceptual material in alchemy is the philosopher's stone, a mythical substance with the ability to turn lead into gold, or bestow immortality and youth. While the search to find such a substance was not successful, it did lead to discovery of a new element: phosphorus.