# 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.