You can save your notebook which contains this function as a .m file then whenever you want call this function, you just type Get["*.m"] or << *.m in your working notebook.
Or, you can save it as a notebook and use opennotebook function in your working notebook.
Hope this helps.
On Mon, Sep 10, 2012 at 10:05 AM, <firstname.lastname@example.org> wrote:
> Hello, > I'm just getting started on Mathematica and trying to define a function > for later use. The function is just: > > test[x_]:=x^2 > > While I don't close mathematica, I can use the function on my notebooks > but when I close it, the function disappers, I tried to save a .m file with > it but It didn't work. What do I do? > > Thanks for time, > Pedro > >