open problem
known problem or question which is thought to have an objective and verifiable solution or explanation, but which has not yet been solved
en-GB/ˈəʊpən ˈpɹɒbləm/
en-US/ˈoʊpən ˈpɹɑbləm/
known problem or question which is thought to have an objective and verifiable solution or explanation, but which has not yet been solved