Unobtainium is a humorous term that refers to an extremely rare, costly, or physically impossible material needed to fulfill a given design for a given application. The name is a blend derived from unobtainable + ium (the suffix for a number of elements). Variations include unobtanium and unattainium, with the same meaning.
The properties of any particular "unobtainium" depend on the intended use. For example, a pulley made of unobtainium might be massless and frictionless. However, if used in a nuclear rocket, unobtainium would be light, strong at high temperatures, and resistant to radiation damage.
