million-dollar question

See also: million dollar question

English

From the TV game show Who Wants to Be a Millionaire?, because in the last question the contestants attempt to win a top prize of $1,000,000.

million-dollar question (plural million-dollar questions)

(figurative) A question that is very important or difficult to answer.
- 2014, Barend Beekhuizen, Rens Bod, Arie Verhagen, “The linking problem is a special case of a general problem none of us has solved: Commentary on Ambridge, Pine, and Lieven”, in Language, volume 90, number 3, page e92:
  The million-dollar question is what mechanisms and representations (potentially unlearned, potentially domain-specific) exist so that the learner, given the input, gradually comes to behave like an adult language user.