Unobtainium

In engineering, fiction, and thought experiments, unobtainium is any fictive, extremely rare, costly, or impossible material, or (less commonly) device needed to fulfill a given design for a given application. 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. The concept of unobtainium is often applied flippantly or humorously.

The word unobtainium is derived from unobtainable + -ium (the suffix for a number of elements). It pre-dates the similar-sounding IUPAC systematic element names, such as Ununoctium.