P.S. I just did a search for "exercise to define the butt" and it brought up a whole list to check out. You might try that.
You don't need to go to the gym. Why not do a search for videos of exercises that will help define the rear? There are a lot of exercises that you can watch online to learn how to do and they are free. There are also dvds that you can purchase from most major stores that aren't too expensive, plus you might check out your local library and see what they have to loan. Once you know the routine, you don't necessarily need the video - you can use your own music if you like.