Unobtainium

"Thus the invention of high temperature fuels will enable us to consider cores which are radiation cooled; such cores would be surrounded by large tanks (cooled by water) which in turn would be penetrated by beam holes. The beam holes themselves would probably have to be made of "unobtanium". (This "unobtanium" is a word coined at Atomics International to describe the material needed to make reactor projects feasible.) In between the radiation cooled reactors and the water cooled reactors is a regime suitable for liquid metal (including boiling liquid) cooled reactors. Once again "unobtanium" is needed."

"The vacuum furnace has been greatly improved and today we are melting titanium, zirconium, high alloy steels and other materials which are remarkably free of impurities and possess improved properties. These high performance metals are used in turbojet engines, missiles and atomic reactors. […] We call it "unobtainium.""

"[D]oes our device require any technologies that we're not likely to be able to develop, buy, or otherwise obtain? This unobtainable content is sometimes called unobtanium, and can take various forms: [...] Things that exist, but don't exist for us. If Acer needs a beautiful custom 15″ LCD panel that costs less than $50, they can get it, because they'll buy millions and millions of them. But for the vast majority of us, asking an LCD panel vendor for such a thing will yield little more than a chuckle."