Fermi problems are questions where rough answers can be estimated from basic principles using little information.
Fermi problems (also known as "Fermi questions" or "Fermi estimates") are questions whose answers can be estimated from basic principles such as dimensional analysis using relatively little information. They are named after physicist Enrico Fermi who was famous for his ability to make quick estimates.
In some cases, Fermi problem may have a rigorous solution using detailed theory and information that is not immediately available, but sometimes when there is no adequate theory or data, a Fermi estimate is the best one can do. Fermi estimates are also very useful in providing reality checks for more complex theory, computations, or experiments.
Fermi problems are often order-of-magnitude estimates, but the majority of questions on Physics SE currently tagged order-of-magnitude are not Fermi problems, but instead use "order of magnitude" in another context, so it is useful to have a separate Fermi-problem tag.
