Getting equation from an unknown curve

Question

So, in my problem I have made use of NDSolve to find the solution of the system of differential equations I have. I was wondering if there is a way to get an approximate analytic expression for the solutions, by using the plots of the solutions. Is there a way to fit a curve in an existing one? I know you can do that if you have a data set. One way around this would be to get the dataset of points, out of your curve and then fit a curve in that dataset, but I was wondering if you can do that directly by only using the original curve.