U is for Unobtainium

Unobtainium (noun):  a fictional material which would be ideal for a particular application however it is impossible to obtain. To clarify, it describes "a material that would be perfect for our purposes, if we could get it, but we can't".

First coined in the 1950s, aerospace engineers used the term when referring to unusual or costly materials, or when theoretically considering a material perfect for their needs in all respects, except that it does not exist. It often pops up in science fiction as a MacGuffin. 

(n.b., a MacGuffin is a plot device in fiction—an object, person, or secret—that motivates the characters and drives the narrative).