I was on an AA MD-80 a few weeks ago, and we experienced a big thump during cruise. It appeared to come from an engine, maybe one, maybe both, but I think it was just the starboard engine. Along with the thump came maybe a half-second power loss (IIRC) and a definite momentary deceleration - it felt somewhat like what you might experience in a car if you hit a big puddle. Everything was normal after that, and the pilot said it was some kind of turbulence, if I understood him correctly. Can anyone elaborate?