(physics, engineering) Any estimation problem designed to teach dimensional analysis or the approximation of extreme scientific calculations: often a back-of-the-envelope problem with limited data.

(number theory) The problem of whether each natural number k has an associated positive integer s such that every natural number is the sum of at most s natural numbers raised to the power k.

(philosophy) A problem arising from the tension between the seemingly irreducible nature of consciousness, and its ubiquity. If consciousness is ubiquitous, and every atom (or every bit, depending on the theory) has a minimal level of it, then it is not clear how these tiny consciousnesses combine to create larger experiences.

(philosophy) A thought experiment, asking, "if a man born blind can feel the differences between shapes such as spheres and cubes, could he, if given the ability to see, distinguish those objects by sight alone, in reference to the tactile schemata he already possessed?"

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

(computing) The problem of how to create a superintelligent AI whose values would align with the interests of humankind.

(mathematics, UK, Canada) The problem in combinatorial optimization in which, given a number of cities and the costs of travelling from one to the other, it is required to determine the cheapest route that visits each city once and then returns to the initial city.

the millennium bug

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(statistics) The bias to meta-analysis resulting from statistical studies with low statistical power tending to remain unpublished and inaccessible to the analyst.