(graph theory) An unsolved problem in mathematics, asking for the largest possible number of edges in a bipartite graph that has a given number of vertices and has no complete bipartite subgraphs of a given size.

An observation that contradicts an accepted law or theory.

A mental representation of a particular problem, including initial, final and possible intermediate states

(euphemistic) A child who is particularly difficult to raise or educate, especially due to a lack of self-control and disruptive and antisocial behavior.

A concise description of a problem

(statistics) The difficulty in drawing inferences from cross-cultural data, due to the statistical phenomenon of autocorrelation.

(mathematics) A theoretical problem in which every nth person, counting around a circle of people, is chosen for execution, and this process repeats after the removal of each chosen person, with the last person left being freed. The problem is to select one's position so as to be the last to be chosen.

(philosophy) The argument that an omnipotent and truly benevolent deity would not permit evil.

(sciences) A simplified problem with clear constraints and parameters, usually used as a tool to understand a related real-world problem.

(philosophy) The epistemological problem that a description of knowledge as "justified true belief" is not adequate: there are counterexamples where an individual has justified true belief of something and yet does not know it to be true.