(mathematics) A mathematical problem, asking whether a room with mirrored walls can always be illuminated by a single-point light source, allowing for repeated reflection of light off the mirrored walls. It was solved in 1958: there are such rooms that cannot be thus illuminated.

(mathematics) The question as to the value of ∑ₙ₌₁ ᪲1/(n²), the sum of the reciprocals of the squares of the positive integers.

(probability theory) The mathematics problem that concerns the probability there exist two people in a random group who share a birthday.

A problem that has been posed in a field of study such as mathematics or theoretical computer science, but whose answer is not yet known.

(programming) A problem dealing with how to treat the initial or boundary values of a discrete problem.

(computer science) The problem of deciding in finite time whether a given program will finish running in finite time, when given a certain finite input.

The problem of finding the motion, relative to one another, of two objects, where the position of each affects the motion of the other.

(statistics) The bias to meta-analysis resulting from statistical studies with low statistical power tending to remain unpublished and inaccessible to the analyst.

(computing, slang) A problem that should be easily resolved.

(calculus) the problem of solving a (set of) ordinary differential equations accompanied by the requirement that the function sought (and possibly also one or more of its derivatives), taken for some value, equals some other specific value; graphically, (for example) the problem of finding that antiderivative of some function that passes through a particular point