Or a tie rod...
To be more specific - grab the tire at 9 and 3 o'clock and try to move the tire/wheel. Do the same at 12 and 6 o'clock. If it moves much at all - you either have A)loose lug nuts (don't laugh, I've seen it) B)worn tie rod C)bad hub (in this case, it would be really really bad)
-Swash