A baseball hit on the ground that takes an unexpected, usually sharp bounce upwards.
While any unexpected movement can be considered a bad hop - off to the side or even some unusual spin on the ball - most bad hops "leap up" suddenly just before reaching the player attempting to field it.
The negative ("bad") nature of the term refers to its difficulty to catch (inherent in catching a ball is successfully being able to predict the ball's exact path or trajectory).
Not only are balls which take a bad hop hard to catch, they are often dangerous and can hit a player in the face or upper body.
Bad hops can be caused by dry or wet spots on the grass or dirt, small rocks or weed clumps, or even tiny imperfections or divits on the surface of the ground.
It looked like a routine play, but the ball took a bad hop off the edge of the grass and hit the shortshop in the eye.