exact solution

In general relativity, a solution of Einstein’s equations is a model universe that follows the law of gravity given by general relativity. If the properties of that model universe can be written down explicitly, for example by expressing the geometry at each point in spacetime with the help of simple functions like fractions, square roots, sine and cosine, the solution is called an exact solution. However, opinions vary somewhat as to what “writing down explicitly” means, so “exact solution”, while a useful term, is not a concept that can be defined precisely.