Bob is a quick up-and-down movement, like when you nod your head rapidly or watch a fishing bobber bounce on the water's surface. When something bobs, it moves in a light, bouncy way, often repeatedly. A cork bobs on waves, rising and falling with the water. You might bob your head to music, or bob up and down while treading water in a pool.

The word captures that particular kind of motion: not a smooth glide or a sharp jerk, but a cheerful little bounce. When you're bobbing for apples at a party, you're trying to catch floating apples with your teeth, and your head keeps bobbing down into the water and back up again.

A bob can also be a short hairstyle that falls somewhere between the chin and shoulders, popular since the 1920s.

People also use Bob as a nickname for Robert, a shortened friendly version of the name rather than connected to the bouncing motion.