Unobtainium is a term used to refer to a material that cannot be accessed. Such a material may be rare, prohibitively expensive, controlled or simply nonexistent. A common observation about unobtainium is that it meets all requirements perfectly, other than not actually existing. Unobtainium is based on unobtainable, substituting the ium suffix commonly used for chemical elements.
Engineers and designers describing the special qualities required in a material for a given product or application, for example, might say that the material they need is unobtainium. The precise properties of unobtainium vary according to the desired application: As a material for a smartphone or tablet screen, unobtainium is extremely lightweight, perfectly transparent, unbreakable, unscratchable and touch-sensitive. As a superconductor, unobtainium can conduct electric current with practically zero resistance at temperatures above the freezing point -- unlike existing superconductors, which cannot. In a hardware context, parts for legacy systems are often referred to as unobtainium.
The exact origin of the term is unknown but engineers have spoken of unobtainium since at least the 1950s. During development of a spy plane at Clarence "Kelly" Johnson's skunkworks at Lockheed, aerospace engineers referred to titanium, which at that time was controlled by the Soviets, as unobtainium. More recently, unobtainium was the material sought in the movie Avatar.
A similar term, unaffordium, is sometimes used when cost is the factor that makes something inaccessible. A cycling enthusiast, for example, might say that titanium wheels are made of unaffordium. The term veryrarium is sometimes used when scarcity is the prohibitive factor.
Today I Found Out explains unobtainium: