Bouncing the ball with one hand (or two hands, if only done once) while walking, running or standing in one spot. It is the only legal method a basketball player can use to advance the ball on his or her own.
Dr. James Naismith, the inventor of basketball, did not envision dribbling when he created the sport. Back then, the only way to advance the ball was by passing it to a teammate or shooting the ball at the basket. Players developed a way to "pass it to themselves" by bouncing the ball and picking it up at a different spot. A dribble in its earliest days was seen as a "controlled fumble".