[eluser]Colin Williams[/eluser]
The class name doesn't matter, but the filename and placement does, if you want it to automatically load. You can of course always do the loading yourself with include() or require(). That's how a lot of frameworks work, but since CI does some handy convention-based auto-loading, many people seem hesitant to load stuff themselves (or they don't know how...)
You can just play into the convention and name your file MY_Controller.php, and CI will load it automatically, and so then your MY_AdminController class will be available, but you might want to keep the one class per file and name it whatever you want. In that case, you need to require it yourself.
Code:
require APPPATH .'libraries/MY_AdminController'. EXT;
class Admin extends MY_AdminController {
function Admin()
{
parent::MY_AdminController();
}
}