The term Unobtanium is also used in SF/sci-fi/science fiction and/or engineering circles to describe a fictional substance with sets of physical properties (density, tensile strength, etc.) that are unnaturally exceptional. For example, an unobtanium garbage can lid that can withstand an artillery shell, yet only weighs an ounce.
This sense of the word is usually used to describe structural or mechanical designs, real or fictional, that play fast and loose with materials science as we know it. For instance, if one wanted to try to justify how Godzilla is able to stand and walk, despite the square-cube law, one might claim that Godzilla's bones are either made of or laced with a very strong type of unobtanium.
One should probably smile while saying that, though.
Of course, YMMV; aluminum, titanium, and steel were considered unobtanium in their days. Even so, if your plans demand unobtanium of any kind to succeed - make other plans.