FAQ: What is closed form?
The solution to a problem is said to be in "closed form" if you can explicitly write the solution in terms of common functions.
For example, suppose that you have two variables x and y, and they are related by f(x, y) = 0. When it is possible to express x purely in terms of y, it is called closed form. For example, y^2 + sin(xy) = 0, you can find closed form solution for x in terms of y, but not y in terms of x.
--Ling Shi, Oct 2006