What does it mean when a music note has an x instead of a circle?

It's like a quarter note, but with an x where the filled in circle should be. When I played cello, we were told it meant to slap your instrument, but does it mean something different for piano? I'm seeing it a lot here. a href="http://www.scribd.com/doc/6013284/BeautyAndTheBeast-BeOurGuest" rel="nofollow"http://www.scribd.com/doc/6013284/Beauty…/a