Science Fiction
Unobtainium can refer to any substance 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. A hull material that gets stronger with pressure in the film The Core was nicknamed unobtainium, but the concept under different names can be seen in the anti-gravity material cavorite and the super-strong material scrith from Larry Niven's novel Ringworld, which requires a tensile strength on the order of the forces binding an atomic nucleus together.
Unobtainium can also refer to any rare but desirable material used to motivate a conflict over its possession, making it a MacGuffin (it appears in the story as something to obtain, not something that is significantly used). An example is unobtanium (sic - per traditional element naming not "unobtainium") in the film Avatar, a mineral valued at "20 million a kilo".
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?").
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