Unobtainium is a term frequently used to describe any material with properties that are unlikely or impossible for any real material to possess and is hence unobtainable. Such materials often arise in the context of science fiction. For example scrith, the material forming the foundation of a ringworld, requires a tensile strength on the order of the forces binding an atomic nucleus together. Since no such material is thought to be possible, a ring world is therefore said to be built out of unobtainium. Unobtainium can be used in a disparaging context (e.g., "that idea is silly; you'd need unobtainium wires to hold the planet up!") or a hypothetical one ("If one were to build an unobtainium shell around a black hole's event horizon, what would happen to the material piling up on it?") The term "Handwavium" is also sometimes used for this material.

The word "unobtainium" is an informal one, apparently developed within science fiction fandom, and probably in ironic reaction to invented element names in, for example, Star Trek (see Treknobabble). It may also be a reference to the naming system for the heaviest actual chemical elements, which tend to start with the letters "Un". In the movie The Core, one of the characters invented a material to build the hull of the craft that dug to the Earth's core - he explicitly dubbed this material unobtainium. Unobtainium also is mentioned as being used in a probability-field weapon in the Uplift Saga by David Brin. See also the list of unreasonably strong materials for more examples.

An earlier source for "unobtainium" exists within the aerospace industry, which has frequently encountered design problems beyond the capabilities of the commonly available materials. Engineers working for Lockheed Corporation at the Skunk works refer to the SR-71 Blackbird as being made of "unobtainium"; not because of the radical decision to use an untried new material, titanium, but because at the time the Soviets were cornering the market in this material and weren't about to let the American military get hold of it. Titanium was required because of the high temperatures that the SR-71 airframe reached. Although titanium alloys have a strength/weight ratio which is much the same as aluminum alloys at room temperature, titanium maintains much of its strength at 600°C whereas aluminum weakens dramatically at this temperature. In spite of efforts by the Soviets a large quantity of titanium somehow found its way to the USA after an apparently innocent European company bought a considerable quantity. The company was in fact a front set up for this very purpose.

Another, more recent use for the term among certain groups is to denote something that everybody could know about (an object that actually exists) but which is very hard to obtain, because of high price or practical reasons limiting availability. It is usually a very high-end and desirable product (e.g., in the mountain biking community, "These titanium hubs are unobtainium, man!").

In maintaining old equipment (electronic and other) it is often used to refer to replacement parts that are no longer made (e.g. many parts for reel-to-reel audio recorders used to play back archive tapes for digitization).