(quantum mechanics) The unresolved problem of what causes a wavefunction to collapse into an eigenstate, or whether wavefunctions collapse at all.

A kind of puzzle in which there is one common factor in a first set of diagrams that is missing from a second set. The solver (possibly a computer program) must identify this factor.

(mathematics) The challenge of proving that the only solution of the Diophantine equation :∑ₙ₌₁ᴺn²=M²; with N > 1 is when N = 24 and M = 70.

Compulsive, excessive consumption of alcohol.

To solve a problem in a way that creates another problem, often of equal nature and severity.

An insignificant problem of a privileged person.

plural of Cadillac problem

plural of teething problem

plural of trolley problem

A type of thought experiment in ethics and psychology, involving ethical dilemmas of whether to sacrifice one person to save a larger number.