Unobtainium is any very rare, expensive, or impossible material needed to suit a particular application.
Engineers have long (since at least the 1950s) used the term unobtainium when referring to unusual or costly materials, or when theoretically considering a material perfect for their needs in all respects save that it doesn't exist. By the 1990s, the term was widely used, including formal engineering papers. (As an example, Towards unobtainium [new composite materials for space applications], by Misra and Mohan describes how the ideal material (unobtainium) would weigh almost nothing, but be very stiff and dimensionally stable over large temperature ranges.)
(via migurski)