Unobtainium

Unobtainium is a term coined by engineers to describe materials that do not exist. When you find that the material properties require for a particular function cannot be met by an material known to modern science, the material that you need is unobtainium. Examples include:

• Metals with impossibly high melting points.
• Structural materials with impossibly high tensile strength.
• Fuels or explosives with impossibly high energy density.

This term has appeared at least once in science fiction, describing a material with impossible properties. May science fiction materials actually fit this description, though.