Unobtainium is a term used to describe any hypothetical 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, 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 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. 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, the hull of the machine that dug to Earth's core was explicitly said by the characters to be made of a material called unobtainium.
An alternative source for "unobtainium" exists within the aerospace industry, which has frequently encountered design problems beyond the capabilities of the available materials. Engineers working for Lockheed Corporation at the Skunk works refer to the SR-71 Blackbird as being being made of "unobtainium" because of the radical decision to use an untried new material, titanium, in the construction of this remarkable aircraft. At the time, "unobtainium" was required because no known material could withstand the extreme temperatures and stresses of exposure to a Mach 3 airstream.
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. Usually a very high-end and desirable product (e.g., in the mountain biking community, "These titanium hubs are unobtainium, man!").
