Definitions
1.NounA fictional material or component which, if it existed, would enable one to easily solve a hard problem.
2.NounAn especially rare material or component that is outrageously expensive or otherwise almost impossible to get hold of.
3.NounA high technology component that is extremely expensive.
4.Noun↑ “unobtainium, n.”, in OED Online , Oxford: Oxford University Press, December 2014; “unobtainium, n.”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.