pl. ; adj. pl. unobtainium; adj. unobtainium-like
1. n., in engineering and Physics, a fictional, impossible-to-obtain material that would be required to build a hypothesized structure or perform an imagined experiment; from un- “not” + obtain “get” + ium (common suffix for elements). Also wishalloy. Ex: But you couldn’t fly the ship into the black hole and survive, you’d need an unobtainium hull.
2. in science fiction fandom, the sci-fi writing community, and role-playing games, refers to any material, object or plot device that explains the ability of characters or fictional technology to perform feats that are impossible in modern science; also handwavium, phlebotinum, eludium, bogosity generator. Ex: “How does the mech’s levitation work?” “I dunno, it’s got unobtainium coils in its flange assembly. Stop asking stupid questions.”
Addendum: This word was apparently invented by engineers in the 1950s, and was later appropriated by the sci-fi community. Unobtainium came to general pop-culture prominence when it was employed as a plot device in the 2009 film Avatar; its use in that movie led to a great deal of ridicule of the term.
“Phlebotinum” was apparently coined by a writer for the television series Buffy the Vampire Slayer.
Other terms used in science fiction and fantasy as a form of unobtainium include, in alphabetical order, adamantium, alicorn, cavorite, cold iron, dark matter, liftwood, mithril, nanotechnology, neutronium, orichalcum, plasteel, red matter, rodinium, scrith, tiberium, titanium-A, transparent aluminum, and vibranium.